If it's in the right slot, and you've restarted your computer, and it still won't work, your USB device driver might need updating. You can either figure out which make/model you have, go to their site, and download your updates there or you can get driver update software that can do it for you. Here's a link that might help if you go the software route: http://www.driverboost.com/lp/drivers/fix-device-not-recognized.php It's not a hard DIY fix, but if you want to save the time or if you're not good with computers then the software might help you out.
I bought a brand new computer that had 2 USB ports that never worked....luckily it had more USB ports....but display USB ports (on a desktop) are usually in the front and are the easy access ones but those do not recognize big USB drives or external drives (another HDD)...
It happens with mine sometimes. Change it to another slot and if it doesn't work, reset your computer. If it still doesn't work, it's probably wrecked.
try the other slot