In a "steady state" configuration where no changes have been made, then disk partitions don't just "vanish".
Here's some thoughts about how I would approach troubleshooting this.
If no changes have been made to the system, then it is reasonable to assume a physical problem, so checking the physical connectivity of the drive - power and data (both ends) - is the logical starting point.
Assuming that this is OK, the next stage is to check it appears in the BIOS (also check the POST messages). Failure to appear here indicates a problem with the power cable (does the HDD spin up?), HDD, HDD controller, cable, or motherboard. Discount configuration issues, as this was a working system until the failure, and no changes were made. Useful things to do include swapping the power lead, replacing the data cable, testing another disk on the interface, etc. Testing the suspect HDD on another system / external USB would be helpful also.
If the HDD appears in the BIOS, next step is to check it appears in Device Manager\Disk Drives. If not, check if the controller appears in the IDE\ATA\ATAPI controllers list. If the controller doesn't appear, the driver could be corrupted.
If it does, then proceed to Disk Manager.
Of course, you can jump in and short-circuit the process at any point.
SD