Assuming that, despite the vast forces involved, the Germans are somehow able to hold the beaches and throw the Allied invasion forces back into the English Channel. What would Eisenhower and the other Allied leaders do if D-Day had failed and the walls of Fortress Europe had stood firm? Would it have given enough leverage to the Germans that they could force an armistice or ceasefire or would the Allies regroup and try again? What would the effects of such a victory have been on morale on both sides?
Eisenhower crafted this message to read on the radio in case the invasion failed. "Our landings have failed to gain a satisfactory foothold and I have withdrawn the troops. My decision to attack at this time and place was based on the best information available. The troops, the air and the Navy did all that bravery and devotion to duty could do. If any blame or fault attaches to the attempt it is mine alone."
Logistically, if the landings had failed the Western Allies would have had serious trouble making another channel landing in 1944. The USSR's Operation Bagration was timed to coincide with DDay. It's possible but I don't think likely Stalin would've called off that assault. IOT, Bagration was a tremendous success and by the late summer/early autumn of 1944 the USSR was very close to Warsaw, Budapest, and beginning to pour into the Balkans. Most importantly it put them in position to begin preparations for their drive to Berlin.
I don't want to speculate on what the Western Allies would've done but a DDay failure would've put the USSR and Stalin in an extremely powerful position after the war, although it would've made the Eastern Front even bloodier as Hitler would've thrown everything he had in the West against the Soviets from that point on.