I think it is a start-up problem.
Do you have the original Install disc?
If you have, insert the disc and restart the computer holding the C key. If it doesn't work do it again and hold the option key. It will come up with a Mac computer image in the center and a circled arrow on the left and straight arrow on the right. Click on the left one first and see what happens.
The software which can fix the problem is the DiskWarrior from Alsoft.com
(approx $100 I am not sure, mine was that price).
I'm really hoping somebody could give me some advice. My friend's Power Mac G4 is broken, and I'm trying to figure how what's wrong/how to fix it. When he turns on the tower the computer sounds as if it's running but it goes to a grey screen with the grey apple logo in the center and the spinning (thinking) gear underneath. It looks as though it is attempting to boot up, but it never goes to the desktop screen. Ideally , we would like to figure out what is wrong and how it can be fixed (cheaply) but most importantly we'd like to know how to get the data from the "broken" computer to an external hard drive (I do have a Mac lap top and external hard drive if this helps). Thank you so much in advance for your help!