It is a very common issue of HTC HD2. After pressing the hangup key, the screen will go unresponsive for several hours or even several days. The problem is caused by the touch screen digitizer which is located directly beneath the hang up key. Those parts are very delicate and can easily get damaged by the frequent pressing of the key.

I think the only solution to this problem is the replacement of the touch screen and the digitizer which will cost around 100USD. If your device have warranty and you flashed an android ROM, then revert back the changes with original HD2 ROM before returning it to the dealer.