Either the enclosure or the drive is broken.
Assuming the drive is in aftermaket enclosure, and not an off the shelf sealed unit, take the drive out and put it in the PC as a second drive (make sure you set the current drive to be a master and the external as a slave using the jumpers on the back. If the drive is recognised, you need a new enclosure. If not, it may be the drive itself.
I'm having problems with my external HD. Running Microsoft XP with a Maxtor HD in a plug-n-play type enclosure. it was working fine up until a few days; just flicked the on switch, autoplay would run, and the HD would be up and ready to go. however, a few days ago, the boot up became rather slow and actually froze up my computer. since then, the HD has not booted up at all. i kno the USB port works because i tested it with an external zip drive and that had no problems at all. i even tried switching out the usb cords, and thats not the problem either. i kno that the computer can detect that something is plugged into the port, but it does not register the usual "remove hardware safely" icon in the tray, nor do the drives show up in My Computer. using the troubleshooter in the device manager, it says that the device is not working properly; that the device "cannot start up". how do i rectify the problem? is this a matter of just getting a new enclosure, or is this something more serious (dead hard drive?). and is all of my data on the HD lost forever, or is there a way of recovering it? i'm hoping its a matter of a bad connection with the enclosure, as others here have posted as a possibility. also, i tried plugging the HD into my laptop and its the same as my primary computer.