When i stert my g4 imac,s screen was bec...

try these articles:
http://support.apple.com/kb/TS1411 also boot to cd and run disk first aid. If it can repair the you have 3 options 1: try 3rd part utility program such as disk warrior or drive genius. 2: try to back up.. you can use target disk mode and hopefully you hard drive will mount, if you dont know how to do target disk mode do a search. if not drive saver can save data but it's expensive 3: erase and install