Energy, normally when you get this type of message:
"Windows detected a hard disk problem. Back up your files immediately to prevent information loss, and then contact your computer manufacturer to determine if you need to repair or replace the disk."
.....it means that Windows has detected bad hard disk sectors. If that's really the case then the drive can never be trusted and should be replaced ASAP.
There are various tools available that can show you the health of a hard drive and PassMark's free "DiskCheckup" is one of them - click here. Install the program, launch it, click on the icon for your disk at the top, click the "SMART Info" tab and look down the "Status" column for any errors.
Let me know what you discover.
"why the hard disk would suddenly have a problem when the computer wasn't even touched. "
Because bad sectors are caused by physical deterioration of the drive's surface which can get worse over time even if the PC's not being used.
PS: Make sure you've got current backups of all your valuable data as there's a chance that the drive could fail catastrophically at any time.