It depends on where the system does not recognize its existence. Does it show in Device Manager but not Explorer? Does it show in the BIOS? Did it have data before, or is it a brand-new drive? The solution will depend on the answers to these and other questions.
The simplest case is if you just installed a second drive in the machine and you don't see it in Explorer; in that case, it's a question of partition and format. If it used to be a good drive, that's another kettle of fish.