Try shutting the thing off then turning it back on after few seconds and see what happens. That most commonly fixes things instead of a restart.
If it boots run Disk utility (Applications folder/Utilties) to check the hard drive for corruption. You can't fix the startup drive bu at least you can find if there's anything wrong. If problems are detected start up from CD and run disk utility.
If the thing won't boot start from CD and run Dik Utiltiy and see what happens. If all else fails, you can always run ARchive and Install from the CD. That give's you a clean OS while preserving all settings, etc. The only problem is that you have to copy a bunch of stuff over from the System Folder (for running OS9).
Booting into OS 9 on the g4 is not a great idea. Running OS 9 apps under the Classic enviroment while booted into OSX keeps things much better. You should not notice any difference in performance when doing this as long as you have a decent amount of RAM.
Using OSX 10.3.8 on a G4