Hardware Problems: If you have external devices such as hard disks or scanners (but not printers or modems) check to see of your SCSI bus is terminated properly. Instructions on how to do this should come with the external device. If confused call your dealer and pester him.
Shut down the machine, disconnect all cables and plug them back in firmly. If they have screws, screw them in. Check for obvious damage.
Beyond this, you will not be able to diagnose hardware failure very well yourself. Try everything in this list and take it to the shop if all else fails.