Gray screen and folder with question mark

I agree with rab777hp, it's probably the hard drive, not the logic board.

First, if the data on the drive is important, boot into Firewire Target Mode (hold down T while booting, whereupon you'll get a bouncing orange icon). At this point you can connect this Mac to another Mac and copy the important files off it. Target Mode will sometimes work even on a very broken computer.

Do you have your system CD or DVD? If you boot to that, by inserting it, and holding down the C key while the computer boots, you can run Disk Utility's Repair Disk function (look for DU in the menus after you get booted to the CD.) If DU can't see your startup disk, then it's toast. If it can see it, that doesn't mean the drive hasn't failed; but try to repair it anyway. Also look at the bottom of the Disk Utility window while you've got the hard drive's first line selected (assuming you can see it); does it say SMART Status Failed? Then you know the disk is bad. But be aware that there are some kinds of failure that the SMART Status doesn't reflect.

Best of all is if you can boot to an external drive with some diagnostic tools on it, or a Disk Warrior CD. While Disk Warrior is rebuilding a directory it can often warn of disk failure.

If it's the stock drive, it's of the age where it's prone to failure anyway. Hopefully you have a backup or managed to recover your files. Replacing the drive is not too hard using ifixit's excellent instructions. Plus this is a good opportunity to upgrade to a bigger drive.