Simply put, they were forced out, ethnically cleansed. Millions of ethnic Germans across eastern Europe (that is to say east of modern Germany) were forcibly repatriated to the reconstituted borders of Germany in the immediate aftermath of the Second World War. This meant that, in some cases, ethnic Germans who had been living in their communities for centuries and had no connection with the German state(s) (see Romania, the Volga Germans, etc) were sent off to a land they knew nothing about and aside from a similar ethnic background and shared language, were total strangers.

This was all done in the name of solving the question of minority rights, which was a major issue in the aftermath of the First World War. The new states formed in Europe in 1918 (Poland, Czechoslovakia, Yugoslavia) had millions of ethnic minorities in their borders, and caused issues for the various dominant ethnic groups. There was no clean way to solve this, short of expelling the minorities (and indeed that happened in places like Greece and Turkey, which saw some 2 million people move between the two countries), so guarantees were sought from the governments to safeguard the minorities, of which there were significant numbers (Poland, for example, was 30% non-Polish at the outbreak of the war in 1939). However this failed to adequately solve the issue, and was seen as a major problem post-war.

While the Germans were not the only group to be shifted around, they were by far the most numerous in this era. This was because it was realised that a major point in Hitler's expansion and annexations was the desire to "reincorporate" these ethnic Germans into Germany, even if they had never really lived there. So the simple solution was to simply have them all removed from these territories, thus removing any future incentive for the Germans to expand and threaten another war; after all, if there are no Germans living in Ukraine anymore, there is no justification for Germany to try and invade there, as it has no claim or kindred folk to "save."

The results was that places like Poland (again I use them due to familiarity) became overwhelmingly homogenous; as noted, in 1939 it was only about 70% ethnic Polish, but by 1950 that number was probably closer to 90%, likely even higher (its currently about 97% ethnically Polish). And as Poland was effectively shifted west in 1945 when its eastern lands were annexed into the USSR and it was compensated with former German territory in the west, this meant even more Germans were forced out, and German cities like Stettin (Szczecin) were Polanised. The same happened with the historic Prussian capital of Konigsberg, which was incorporated into the USSR and renamed Kaliningrad. It's German character was stripped, and a new Russian façade was created.

In short, the German population of these regions was expelled from the region over the course of 1945-50, so that these regions became Polonized and Russified. A good portion of the region's population fled during the war because of the advancing Red Army, but there still was a sizable population remaining after hostilities ended. Allied wartime agreements at Potsdam sanctioned population transfers into occupied Germany, and the process was staged over the course of five years. The Königsberg region of East Prussia, which became the Kaliningrad oblast, had most of its remaining German population expelled between 1947-48. Despite Potsdam's assurances that transfer would be "orderly and humane," the expulsions were often poorly organized and quite inhumane. Many of the expellees lost a lot of their possessions and became refugees inside Germany.

Although some expellees settled in the Soviet zone, a good many of them ended up in the Western zones. They were often sheltered in temporary housing, such as the Dachau barracks and later prefabs. The fact that the communist governments had conducted the expulsions added to their sense of victimization. This politically-charged sense of identity meant that the expellees became an important voting bloc in the Cold War FRG. In the early decades of the FRG, most of the mainstream political parties sought to court this voting bloc and keeping the issue alive was one of tactics used to gain their constituency. School maps, such as this one- note it also labels the GDR as "Soviet occupied zone"- and other materials would label these areas not as Poland or the USSR, but rather as "under Polish/Soviet administration." Early electioneering in the FRG also built on this sense of loss, such as this CDU poster edit clearer link or this 1949 SPD poster. Even though the CDU and SPD were on opposing ends of the FRG political spectrum, they sought to mobilize the expellees as voters by holding out a shady promise of reuniting the lost territories with Germany. Even in the GDR, SED local leaders told their rank and file members that while the Oder-Neiße line was an immediate reality, dealing with the Soviets and Poles might allow for a more equitable rearrangement between socialist brothers. In private, nearly all of the FRG's politicians recognized that gaining this territory back was politically impossible. Adenauer, whose Rhenish background made him particularly hostile to Prussia, did not shed many tears over losing East Prussia and Silesia. In a 1953 conversation with West Berlin Mayor Willy Brandt, Adenauer would exclaim ""The Oder-Neiße, Eastern provinces - they're gone! They don't exist anymore."

The expellees also created various special interest groups and organizations that kept the issue alive during the 1960s and 1970s as it became more apparent that the borders were fixed. Although some of these expellees were revanchist, most of them recognized that this territory was gone. Tours of the lost East organized by these groups in the 1960s and 70s often underscored the permanence of this loss. Andrew Demshuk's recent study of these groups persuasively argues that as new immigrants to the FRG, the amount of attention paid to the Heimat of the East was a mechanism to cope with their internal migrant status by creating a "Heimat of memory" to assert their German identity and heritage in the FRG. These expellees were internal migrants into an country which prided itself on extreme localism and regional identity was often indelibly mixed with the conceptualizations of national identity (the "nation was a local metaphor" to paraphrase Alon Confino). Emphasizing the essential Germanness of these areas made the expellees more German and less foreign in this context. While there the expellees' publications had prominent anticommunist editorials about the how former German land was now under a brutal occupation, the rest of the articles focused less bellicose matters like forging new associational connections within the FRG.

The superficial nature of this revanchism was really in evidence during the reunification of Germany and the negotiation of a final peace treaty. Kohl pretty much did not hesitate to affirm the Oder-Neiße border as the price of unification. The Soviets may have offered Kaliningrad to the Germans as compensation for Soviet debt, but if such offers were made, the German negotiators did not really bite at them. The Expellee associations themselves are a bit adrift in the unified Germany, with no core issue left, declining membership, and an unsavory association with various right-wing German politics.