Do you run XP? If so, try running a scandisk. I had a couple of weird things like this, where something was blank, someone told me to run a scandisk and after it was done, everthing was there.
For Windows XP-- Start>My Computer>right-click on "C" drive>properties>tools>check now. A window will pop up with two check boxes, tick both of them, then okay. It will tell you it needs to restart to start the scan. So, reboot and let it run. It takes a while and then the results just flash there for a second, so if you're not watching you'll miss it. Then go and see if the device manager is back.
Hope this helps, I'm not very experienced with computers, but this does sound like a problem I had and this is how it was fixed.
At any rate, doing a scandisk isn't going to hurt anything.