Powerbook G4 will not boot up

Please help. My powerbook G4 will not boot up. A few months ago I added a 1G ram and most recently with in a few weeks I added an upgraded 250GB harddrive. Just the other day I went to turn on my mac and it only displayed the grey/blue screen with the apple logo and the loading sproket. I've let it run for 30mins like that, i've pulled the battery, I've replaced the ram with the old 256, I've tried to use disk ulitily and first aid which said that the "volume harddrive appears to be ok", i've tried to do a safe boot and nothing. Ever re-try leads me to the same result of that first screen.

Will someone please help me as this is my first computer and I am learning so much from the experience. I am fearfull to lose my data as well. The ironic thing is that the day I went to turn it on was the day I was going to back it all up on my new time capsule. Please, I would love anything. Thanks

Can you boot from the Leopard install DVD? If so then run Repair Disk from there. Keep us posted.

I've tried more things to no avail

I've loaded another leopard disk to go into disk utilities. I've verified disk permissions, repaired disk premissions, repair disk. I've reset the pram, nvram and pmu. When this happened I was doing an update for my laptop when I had to leave in the middle of it so I just unplugged everything and put it to sleep by closing the lid. It was taking a long time to do the update at the time so i'm wondering if that might have been the problem. I don't know what to do from here.