Are they seen on other computers? My first guess would be that your USB port is dead or switched off.
Try some of them in another machine. Try the simplest USB memory stick in this machine and see if it works.
And attach one of the drives to this machine and see how they show up in Device Manager and possibly Disk Manager.
Check disk manager. My external backup drive sometimes doesn't show up in windows 7 because the OS forgets the previously assigned Drive letter. I just have to reassign it to G: and it works fine...until it forgets again a few weeks later.