try this.
in the game you're trying to run, right click on it in steam, go to properties. then, click on "set launch options". this box allows you to enter in commandline codes the game will use when it is run. type in "-sw" without the quotes. now try to run the game. it should be forced into a window'ed mode.
he probably set the resolution to below what your monitor/gfx card can support. however, windows don't force the computer's resolution to change, instead only changing the size of the window itself. so whatever resolution it is, the window should be able to play it. then, change the resolution to whatever you want.
alternatively, if that doesn't work, you can get rid of the -sw command and replace it with the following (w/out the quotes of course) : "-w <width> -h <height>" where <width> and <height> correspond to your normal resolution. those commands are supposed to force the resolution to that size.