hey--been a while since I used one, but if I remember correctly....there's a "white light" lever on one side of the head (right side?)---you can flip this lever in one direction to swing the filters out of the lightpath to focus. If the filters are out of the lightpath, then the head won't switch on--basically to save you from wasting paper. Normally you'd go to whitelight to focus, then switch the filter pack back, then hit the timer to expose. IF you forgot to flip the lever back? the lamp wouldn't come on, but everything else would be powered up.

could be something else, but this would be the first thing to try.