With the USB key removed, reboot. Then replug the USB key. And if it is seen by Windows, before loading anymore stuff on it, run a CHKDSK command on it.
I am having the same problem with a SanDisk 32 gig usb stick (cruzer fit) - it worked for a while and now it does not work now. I went through all the steps listed above as well. While using DISKPART this is what I got from the “Clean” command:
Same happened to mine. It was almost completely full then everything on it disappeared during a file transfer. I turned on every windows service that could effect it but nothing would work. I took it from my usb 3.0 slot and inserted it in a usb 2.0 one and it actually read it but some files were locked or something. I also found on one of my free download sites a file unlock utility made by iorbit. Your files are there just keep messing around with it til you find them. Options are to try and back it up, complete copy, defrag can help(if it lets you) uninstal/reinstal driver, or any utility program that can help. Between advanced system care(iorbit), ccleaner, and glarys utilitys there is not many programs I need.
Are you referring to the 30 day free trial version or the $29.95 version?? The similar Slimware Utilities Slimcleaner Free is rated better by PCMag.com and is completely free.