Check all connections are good and try another power supply in case it is this. You could also try another USB cable as these sometimes fail.
It may also be a bad connection to the drive inside the caddy check these also.
If none of the above work you may have either a HDD or caddy electronics failure.
To check you could remove the drive from the caddy and install it into your PC. If it works there it may be the caddy that has failed - you could try another drive in it to see.