Before you panic, take another stab at booting. Maybe your cold hard drive simply couldn't answer the call in the early chill of morning. If that fails, open the case and check the power and data cables. A loose connection can prevent your system from seeing the drive.

Did you recently install a new drive in your PC? If so, check the dip switches or jumper pins on the drives to ensure that the master or slave settings of the IDE devices don't conflict.

Worst case, your drive is dead or unbootable (the critical boot sector of the disk may be damaged). If this is your boot drive, you can use a floppy- or CD-ROM-based utility to boot the PC and investigate the problem. Or take the drive out, set it as a slave, and plug it into a working PC to test it. If it works, pull your critical files off the drive and reformat it.