Please try the following:
- Disconnect all cables connected to the drive.
- Connect the power cable to the drive.
- Connect the USB-cable. First to the drive. Then to the computer.
If the drive is detected correctly the problem was probably caused by improper shutdown or un-mount (unplugging the cable without dismounting first, etc.). When this has happened, the USB-interface needs to reset itself properly. After this, reconnection should be no problem.