I had the same problem, a couple of weeks ago, with two USB external drives that had been absolutely fine for several years. Windows reported no problem when I eventually managed to persuade it to read them again, but backup failed part way and they again became "unreadable". Powering down and restarting brought them back.
I suspected the Windows backup routine might be at fault, so I downloaded Paragon Backup 2010 Free. That told me that there was a problem with both drives and that they needed to be reformatted.
I deleted and recreated the partition on each, reformatted them, and they worked again fine. Paragon also says they are OK now. It was odd that both went offline the same day, after years of flawless operation, and when neither had been "used" for some days.
I don't know if a Windows 7 update did something - I seem to get an awful lot of updates these days.
I worried that they are on their way out, so I bought a brand new (fast) Verbatim 1Tb eSATA drive, but I find to my annoyance that eSATA does not work with Windows 7, so I am having to use the drive as USB. A different problem.
My suggestion would be: if there's stuff on them you need, back it up to another drive first, then repartition and reformat them.