Erm, could be a lot of things (well 4): 1) Its broken 2) Its not plugged into the computer properly 3) You have turned down the brightness 4) You have fiddled with the contrast. Try seeing whether it is plugged in OK then going through the brightness/contrast controls before trying the monitor menu controls then finally facing the awful truth that you need to buy a new monitor.
Normal color settings on yer control panel is 256....may have been changed (reduced) accidentally? When booting up press F10 a few times and you should get choices menu appear. Go for DOS setting. Cakky set up, but it allows you to go into settings again and alter colors back to 256. Re-boot ..... and Bob was my uncle when similar happened to me. Worth a go, rather than writing it off as bust.