Most land in Medieval Europe was owned by lords and they would fight over land and all that stuff. What would happen to a lord if they lost control of their land? Do they become peasants or do they get executed, what happens to them?
This is a humongous question and I don't know that there can be any very general answer, but I can give three examples, from the Douglas family in Scotland, that cover at least some of the possibilities. The Douglases were a noble family before the Scots War of Independence, initially holding lands in Lanarkshire in the south-central Lowlands; later they became great territorial magnates in the west of Scotland, and held so much land that they are the go-to example of an "overmighty vassal", a lord more powerful than the king - which became their downfall, as the kings of Scots naturally maneuvered to reduce their power.
The first example of losing land, however, comes when the Douglas lands are still only a barony, which William "le Hardi" Douglas lost by joining William Wallace's rebellion; he died in the Tower, and his son James didn't inherit, since the lands had been confiscated. James (who'd been in France for safety) tried to petition King Edward to be allowed to inherit his father's lands, which didn't work. Instead he joined Robert the Bruce's rebellion, where he became a famously ruthless guerrilla commander, getting the nickname "the Black Douglas" for his policy of terror attacks on the occupying English troops. So that's two possibilities for a response to losing your land: First, you go to whoever took them and ask nicely if you can have them back; if that doesn't work, join their enemies. It sounds ridiculous put like that, but it's not completely silly - note that James isn't asking for the return of lands confiscated by main force, but to be allowed to inherit lands that his father lost by rebelling. He's asking for a favour, sure, but not for pure charity, because he's offering an implicit quid pro quo: Let him inherit, and he'll make his peace with the English king and not do exactly what he does next - join the rebels. A nobleman whose family has been in an area for some time isn't helpless because someone has written words on paper saying he doesn't own such-and-such an estate any more; he has legitimacy and (unless the family were seriously unpopular) local allies. That doesn't necessarily matter in peacetime when the King sits securely on his throne, but of course, that's not when people are likely to lose their lands in the first place.
James Douglas was Robert the Bruce's most effective field commander, and was rewarded accordingly when they won their war; he was created Earl of Douglas and given lands taken from the Balliols, and the Douglas family were powerful and important vassals in Scotland for the next hundred years. Too powerful, in fact; by the fifteenth century, the kings of Scots were trying to reduce what had become almost a second state within Scotland. At the "Black Dinner" in 1440 (incidentally, when George Martin named the "Red Wedding" he wasn't being particularly original) William Douglas, the 6th Earl Douglas, and his brother David were beheaded, and as a result, the lordships of Annandale and Bothwell fell to the crown;their great-uncle James Douglas became the 7th Earl. In this case the loss of land was caused by death, so there was very little the victims could do about it. You might ask, of course, whether James Douglas wouldn't want to get back the two lost lordships for the family, and perhaps he did; however, he had conspired to arrange the Black Dinner so he could inherit the Earldom, which made him the most powerful man in Scotland (King James II being ten years old at the time, and under a regency) and also united the lands of two separate branches of the Douglas family. So you could regard this as another possible response: The lands in question were lost as part of a gambit to gain power within the kingdom, which worked magnificently - James could compensate himself freely for the loss by all the means available to a man who is, in effect, Regent for a child king.
That worked only for a while, however. Eventually James II came of age, and decided to claw back some of what the Douglases had grabbed while he was underage; in 1452 he personally killed the 8th Earl, William Douglas, after quarreling with him over an alliance he had allegedly formed with John McDonald, Lord of the Isles. This started a period of on-again, off-again civil war in Scotland, lasting from 1452 to 1455: Every so often James would declare that such-and-such a bit of land was confiscated, the 9th Earl (yet another James - Scottish history could be a drinking game) would rise up in rebellion, there would be an inconclusive battle and King James would back down. This is yet another means of responding, available only to very powerful lords: Start a war. (Not the same as joining a pre-existing rebellion). In each case the King would have to back down because the Douglases could find a lot of allies among the other noble families, who weren't by any means happy to see the King confiscating land - if he could do it to the Douglases, then why not to other noble families? The nobles of Scotland had, you might say, enough of a sense of their class interest, as against the King, to band together on the principle of "First they came for the Black Douglases, and I didn't speak up..."
However, James II was nothing if not persistent; he managed to dislodge several of the Douglases' allies by giving them lands, titles, and offices - bribery, in other words. In 1455 the cycle of confiscation, rebellion, battle occurred again, but unlike the previous fights Arkinholm was a clear victory for the King's troops; two of the three younger brothers of the Earl died, and the last fled to England. Shortly thereafter the Scots Parliament declared the Douglases attainted and all their lands forfeit to the crown; much of that land was then handed out again to the king's allies, as thanks for helping him out with the Douglases. However, Earl James had gone to England before the battle to look for support, which brings us to the final option I'm going to mention: Bring in some foreigners. In 1461 we find him trying to raise rebellion in the west of Scotland, with English support, to topple young James III (take a shot!) and, presumably, become an important figure in an English-backed regime, perhaps in effect a vassal King of Scots. That didn't work; but as late as 1484 he invaded at the head of 500 English cavalry, hoping to raise yet another rebellion against James III. This time the local militias instead came out in favour of the King, defeated the Earl's force and took him prisoner; he died in prison four years later.
Counting up the responses, we have:
Option 3 is perhaps more common than the history of the Black Douglases makes it look; the later Earls were immensely powerful, as feudal lords go, and had options that would not be available to the average noble. Similarly, lawsuits are also more common than civil wars even if the Douglases would make you think otherwise; for that matter, the Douglas history also contains several lawsuits (usually over inheritances) that I've skipped over because those land disputes were internal to the family. Still, I hope this gives you some idea of the range of possibilities.