I encountered this with some games randomly occasionally (but not often enough to report) and found that input would work once again when alt-tabbing away from the game and then back to it. Have you tried that? Also, is there anything else that seems unusual when you arrive at the screen where you can't give input? For example, nothing on the screen updates when it's supposed to, or you do/don't keep hearing sound? Also open a CPU monitor while running the game and check how the screen affects the CPU compared to before you got there. Taking all this, you could determine whether the game just crashed, or if there's really an input problem. This should probably just be obvious at first glance. (By the way, you can usually use Alt+F4 to quit any Windows game, even if the game itself has no more input.)
