With your flash drive plugged in, try this: go into the Device Manager console (if you know how) and find the entry pertaining to your flash drive under 'Disk drives' or similar. Right-click the entry and select 'Uninstall' then you can do one of two things: either click the button at the top entitled 'Scan for hardware changes', or restart the computer. Either way Windows should then try to re-install the driver for the flash drive and then mount with a drive letter for access.
Failing this try running the Windows error checking utility for your flash drive and see if anything comes up.
Hope this helps.