Clarification/Follow-up by Jim.McGinness on 05/29/07 4:26 pm:
If you say "all of a sudden", I have to assume you've made no changes to the BIOS. It's possible to disable the hard drive controller via the BIOS on most motherboards. You can check for this, and also check for loose connections on drive cables. In my experience, these connectors don't simply work their way loose, any more than isolated BIOS settings change by themselves.