im a w850 and most of my friends are and im going to bet you need the top keypad replaced. it seems that the top keypad was designed badly in the w850 and therefore cant with stand a lot of wear and tear.
ie. i doubt its firmware or the ribbon, most PROBABLY topkeypad.

um...besides the white screen, can you use the phone normally? do you see a blinking red LED when you try turning it on?