Try it on another computer. If it's not recognized there, you'll need a professional data recovery firm to work on it.
If it is recognized, copy the contents to another location IMMEDIATELY.
Then, run a hard drive diagnostic on it to see if it's failing. Try this free program: FWIW - here's a free generic test that runs in Windows (select HDTune, not HDTune Pro): http://www.hdtune.com/download.html
If it is recognized, copy the contents to another location IMMEDIATELY.
Then, run a hard drive diagnostic on it to see if it's failing. Try this free program: FWIW - here's a free generic test that runs in Windows (select HDTune, not HDTune Pro): http://www.hdtune.com/download.html