I have a 1TB external hard drive. It use to work really well and all the time, but just recently my Mac won't pick it up when I plug it in. What should I do to fix this?

Does the HD appear in Disk Utility?

Does the HD appear in System Profiler

OS X: About System Information and System Profiler - Apple Support

under the type of connection (like USB). You should see both the type chip in the enclosure as well as the actual HD.

Have you tried another cable and port on the computer?

Does it appear on another computer?

Do any light lite on the enclosure?

Can you hear/feel it spinning?

Is it powered by the USB bus or have separate power supply