Plug in the flash drive, then launch Disk Utility.
/Applications/Utilities/Disk Utility.app
See if you can repair the flash drive. If Disk Utility can't repair it, then your choices are to reformat the drive (this will erase the files you want to recover) or use a 3rd party recovery tool.
Do you know what format was used on the flash drive? ie: HFS+ (Mac format), FAT32, NTFS (Win formats)?