: reboot your pc, link your usb drive to pc again, if it dose not work, try another computer. Sometimes this will work, if don't, try solution two.
: download a free data recovery software, restore your files to another usb drive. Generally speaking, all your lost files will be recovered.s
: when the file recovery is finished, you'd better check your usb drive.
1.Link it to your pc, right-click on your usb drive letter in "my computer"
2.Select "property", find "hard drive" and select your usb drive in"name", hit "property"
3.Select "strategy" and then "optimization" in the property dialog.
4.Wait till the optimization is finished. Hit "Confirm" and reboot your pc.
5. Format your usb drive.
If solution three dose not work, perhaps your usb chip is damaged, buy a new one.
check out this link too . http://www.brotsky.tv/2014/05/18/how-to-save-a-file-to-a-usb-drive/