|Seems to me that the partition on the flash drive corrupted. the best way to retrieve the files is to run a data recovery program. Ontrack DataRecovery is a best program that recover any RAW data. you can download the trial version from www.ontrackdatarecovery.com|
Or find someone that own the program, I know Hiren's BootCD have it included. do a google search on it.
And I believe 99% of your files still remain on flash drive, unless you try to overwrite it. hope it helps