My guess is that you need a new display. But it is quite easy to dismantle the N900 if you have the right screwdriver (here is a blog with a link to the disassembly service manual there may be better links out there, but that is the first I found) you just need the small phillips screwdriver to get the screen off, you only need the small torx driver to dismantle the base. So I suggest you take apart the screen and make sure all the cables are firmly together, if that doesn't help you probably need a new LCD.