You give very little info but here goes, there can be a number of reasons:
1. The driver software is missing, damaged or affected by a virus;
2: The drive itself has become damaged (ie the motor does not run).
3: The power cable to the drive has become detached at either the drive or motherboard end
4: Ditto to 3 but for the data cable
5: The electronics of the drive have been damaged.
6. The drive is not being recognised by the BIOS.
7: The drive is not receiving any volts from the power supply
If the drive goes round then the electronics of the drive are probably OK. It would then be a software (driver) type of problem.
Try borrowing another drive and see if that works. If not then it is definately a computer problem, (either volts or software)
If that drive (known to be working) works then your drive has bit the dust!