You might try different USB ports, just in case the one you are using is not functioning properly.
Also, when you have it connected, go to Control Panel–>Administrative Tools–>Computer Management–>Disk Managment. See if it shows up in the list of drives attached to the computer. If not, there is some type of hardware problem.
Look in Device Manager and see if there are any yellow caution flags for the drive.
If you have access to another computer, see if it will show up in it.