Just to confirm the obvious, have you tried forcing it to turn off (by holding down the power button for five seconds), then turning it back on again? On rare ocassions I've seen Macs get stuck at that spinner for no apparent reason, and it can't hurt to give it one more shot.
Past that, get either the DVD that came with the computer or your newest OS install disc, and boot up from that (hold down the C key, I believe, when you first turn it on with the disc inserted). Once booted from the disc, select "Disk Utility" from the menus, and do a check on your drive. If there are errors, let it fix them, and keep running it until no more errors show up (if it keeps coming up with errors after three or four runs, you can give up, though). Also have a look at the part that says "SMART" down at the bottom--it should say "Verified". If it says anything else, your drive is dying.
Presuming that's not the case, try starting up after the repairs. If you're lucky, it'll work. If not, you should probably try doing an "archive and install" of the OS. That will, unless the drive really is dying (signs would be making odd noises, running very slowly, or errors that Disk Utility can't fix), get you back up and running without (in theory) losing any data.
Of course, the FIRST thing you should do in any case is back up anything you don't have backed up yet--mounting the computer using Firewire Target Disk Mode connected to another Mac is the easiest way to get at the files.
