I was going batty because for the last few weeks Dosbox would run and only "display" a completely black window that would not react to any keys. Nothing helped. I finally ran it with the console window open and noticed a line about "using MS joystick"... and I remembered that a couple of weeks ago I plugged in a normally unused, dusty joystick into the Sound Blaster joystick port... I unplugged it, ran DB - and it worked perfectly once again.

Perhaps it's only my system, but perhaps you should look into some potential trouble between DB and real joysticks...