|Is the drive recognized by another computer?|
If not, it could be either the hard drive inside the caddy, or the caddy itself.
I've had a number of USB caddies go south on me. The hard drive inside is usually okay, and can be slaved to your machine, if you need to access the data.
If you have another hard drive lying around, you can put it into the caddy to test it.
If you find the drive is okay, but the caddy is not working, you can buy an empty caddy fairly cheaply and place your 500GB drive inside.
Please let us know if you found someone's advice to be helpful.