First thing to check is whether it is still showing in BIOS. If it is recognised in BIOS, then it could be a software (e.g. Windows) issue.
If it isn't showing in BIOS, you need to remove the cover on the computer and visually check its data and power connectors haven't for some reason dislodged.
Did you move it, or drop it, or do anything with it PRIOR to this happening?