I had the same problem when I accidently forgot to safely remove hardware, and came across this post. Here's what worked for me with Windows 7:
1. Click start, go to Control Panel, then Device Manager
2. Right-click “USB Mass Storage Device”, and elect “Uninstall”
3. Unplug the Flash Drive and wait a few seconds
4. Plug it back in
5. Windows will give you the same
“disk not formatted” error - don't worry...
6. Do not format the drive – click cancel
7. Then safely remove hardware (like you would normally)
8. Remove the flash drive, wait a few seconds, then plug it back in.
9. Now that it has been
“safely removed”, Windows should give you a message saying “do you want to Scan and Fix” – Click the “Scan and Fix” button.
10. Check
“Automatically fix file system errors” and
“Scan for and attempt recovery of bad sectors”
11. Click “Start” to start the scan, wait for it to complete… and then pray that it works for you like it did for me

Good luck!