Do you have a network drive mapped on your PC? Quite often, Windows will give a USB drive the same drive letter as a network drive, in which case the flash drive cannot be accessed until either the network drive is disconnected or the flash drive letter is changed.
Just in case, try carrying out the following steps with your USB drive plugged in (based on Windows XP):
- Go into the start menu, right-click on "My Computer" and select "Manage"
- Go into "Disk Management" on the left and look at the drive listing on the right. E.g. if it's a 4GB drive, look for the drive around this capacity. If you don't see it, then there's likely a more serious problem with the flash drive.
- Right-click on flash drive in the drive list and click "Change Drive Letter and Paths...", then click the 'Change' button in the window that appears.
- Give it a new letter that is not used by any other drive, e.g. "T".
- Click 'OK' and accept the warning that appears.
- Go into "My Computer" or wait a moment to see if the Autorun takes place.