Blue Screen with spinning coloured wheel

My girlfriend borrowed her iMac G4 Flat Panel to a friend and now it won't load the OS properly.

The following happens:

1. The grey screen appears (with the smiling MAC in the middle)
2. The Mac OS X start-up screen appears with the progress bar
3. Then a blue screen, with a spinning coloured wheel in the top left corner
4. After a couple of minutes, a few windows pop open (probably whatever the person accessed last)
5. The windows disappear and all you're left with is a blue screen and spinning coloured wheel in the top left corner
6. The MAC then remains in this state indefinitely

Unfortunately, I’m unable to tell what version of the MAC OS is running.

We cannot find the OS disk either.

I will greatly appreciate any assistance anybody is able to provide.

Thanks

Thank You for the link.

The MAC is back for the time being.