Upon restarting my Mac G5 (it was a normal restart, it wasn't frozen or anything), the grayish-white screen came up that normally ONLY has the little "thinking" animation that looks like little lines organized in a circle pattern denoting OS X loading and the apple logo. Well, this restart had all that, except in place of the apple logo was a "No" symbol (like No smoking...) and it stayed on that screen seemingly indefinitely. So, I restarted again, and it was fine. So, everythings fine, but I was wondering if anyone out there knows what it means when that happens. Any thoughtd? Thanks!
It can't find a startup drive/OS. Something got corrupted (most likely) or the startup drive has failed.
I would startup off of the the installer disks that came with your Mac. You can then try repairing your startup drive using Disk Utility (found on the installer disk pull down menu).
If that doesn't fix the problem (try restarting again of the internal drive after repair) just start up off of the installer disks again and do an "archive and install" to replace your existing OS with a clean install (you will need to run all the updates again afterwards). Be sure to turn on the options in the Archive and Install options to save your files, users, etc (can't remember the exact terminology).
Its always a good idea to repair your permissions and repair the drives (using Disk Utility or a 3rd party application like Disk Warrior) before and after running any major software updates and/or every month or so, to help prevent problems from occurring in the future.