Welcome to the forums, thanks for your question, now can you supply us with even a hint of detail? It can be only 1 of about 50 things depending on many variables. Are you talking about the BIOS, or in Windows? What is your motherboard? What is the drive?
If it's a drive that worked before, and has data that you want to access, we need to know just how far up the line it can't be seen, again as JIT wrote. Is it detected in BIOS, in the device manager, in the disk manager?