Check out the book 'The Longest Day' by Cornelius Ryan. I'm unfortunately traveling and don't have it in front of me, but if memory serves, Hitler was convinced (thanks in large part to Allied deception campaigns) that Normandy was a diversionary attack.

The Allies had been heavily bombing the area around Calais, farther east along the coast from Normandy and at the narrowest point of the English Channel, in an effort to convince the Germans that the main attack was to take place there. It worked. German reconnaissance aircraft were unable to spot the Allies' troop concentration in England, due to Allied air superiority, and so all they really had to go on was the intensity of 'preparatory' bombing in various sectors.

In the morning of D-Day itself, the German 7th Army, based in Normandy, found itself under attack by paratroops and seaborne infantry. Reports and communications amongst the Germans at this time were fragmented and contradictory. The paratroops has helped to spread confusion, the French underground had been ordered to cut telephone cables in the area, and Allied air attacks hindered communications. Additionally, Field Mashall Rommel (whom Hitler had placed in charge of preparing to meet the expected Allied invasion) had only two days before returned to Germany on leave, and several other local commanders had left their headquarters before dawn to attend war games scheduled for the day of the invasion. For these reasons, it took a while for the gravity of the situation to filter up to the high command.

Nonetheless, von Rundstedt, the overall German commander in France, immediately placed on alert two reserve units -- elite Panzer and SS formations -- in preparation for them to meet the Allied attack. Technically, these units came under the direct control of Hitler's headquarters in Berlin, but von Rundstedt had no reason to believe that Hitler would object to this.

He was wrong. Later in the morning, von Rundstedt placed a call to Jodl, Hitler's chief of staff, to formally notify them that the two reserve units were going into action. Jodl was furious and ordered them to stand down immediately, probably due to Hitler's conviction that the attack was going to take place at Calais. Hitler himself was at his private residence, and by Ryan's account passed an uneventful day. So convinced was the German High Command that the Normandy invasion was a diversionary attack that the 15th Army, defending Calais, was left in position until late July. (The invasion took place on the 6th of June.)

So to answer your question, Hitler's immediate reaction was unconcerned. He and many others in the German chain of command did not immediately believe that the Normandy invasion was the Allies' main thrust.