Can Anyone help, my computers just come back from the shop, they put in a new power supply, I get home and my second hard drive isn't showing up, it shows up in device manager but not my computer. Please help

If you can see it in device manager, with no yellow ! Marks, then Windows is recognizing it correctly.

Go to disk management by typing at the "run" prompt: diskmgmt.msc.

This will show the available drives and the current letters assigned to it, and their status.

What does it say?