Had a very similar problem with a 2 day old wildfire S. (As soon as I opened the box,I swapped the SD card that came with it for a bigger one I had). All OK for 2 days. Then, for no apparent reason, the phone backlight would not come on AT ALL after waking it from sleep. (phone was still on as I could call it and it rang) If battery removed and reinstalled it would come on and behave until the screen went out in the normal way from lack of activity. Then it would not light up at all (apart from the 'buttons' along the bottom). Only solution was to take the battery out again! I tried this many times and uninstalled lots of software, but always the same problem.
I did some research in these forums (for older wildfire) and tried changing the SD card back to the original (which did seem to have a tighter fit than the one I installed) This, so far, has completely solved the problem. So, if you have this problem, try changing the SD card!
Hope this helps someone.