next time you boot it up, press command v when you hear the startup chime. This will start printing a lot of status messages to the screen.
If it hangs, note the error.
If you have good, steady hands, or a tripod, you can photograph the error for future reference. (I really shouldn't have to say this. but blurry pics are, of course, useless).
You can also boot the mac into target disk mode, and look at
var/log/system.log on the powermac's drive