|If it is being detected by the BIOS, then the partition or allocation table may be corrupt.|
Sometimes chkdsk /f will fix it. other times requires the use of a utility to get your files. These are not 100%. It requires putting the ailing drive in a system with a working drive to copy the files to. Recuva is one of many apps to do this. EasyRecovery is another.
If the drive is not detected by the BIOS, the drive heads may have crashed into the platters. In this instance, it's gone.