I recommend you try some basic troubleshooting:
1- Disconnect the drive from the computer and restart the computer.
2- If the computer is a desktop, connect it on the back USB port (directly to the computer, not through a hub).
3- Go to "Disk Utility” to verify if the drive shows up there.
If the drive shows up in Disk Utility but it is not seen on Computer, then it means the partition is corrupted and you will have to format it in order to use it again. Please note that formatting the drive will delete all data stored on it.
It may be possible to still extract the data utilizing data recovery software. There are several kinds of Data Recovery software available. If you do a web search for Data Recovery it will yield plenty of data recovery software options.