Not reading Drive C

I had a user that was accessing files from his computer and for no apparant reason the PC would freeze up and a blue screen with the following comment “Not reading Drive C” would appear. If you hit enter then a black screen would appear, but you could nothing with the computer except hit the reset button and start all over. The computer is a Dell Optiplex GX1. I know there is a right and wrong way in fixing a problem I would appreciate any help that any body would be able to give me. Thank you for the help.

Markus R Bertel