It is possible the partition is corrupted. As a troubleshooting step I recommend you try replacing the USB cable and also make sure the drive is plugged in directly into a USB port and not a USB HUB.
If this does not work then you may try formatting the drive in order to use it again. However, this will delete all data stored on the drive. If you wish to recover the data, try using a data recovery software before trying to format it.