Hey amedinaok. As Someone Somewhere mentioned, a bit more specific info is needed here. Though you could try to run chkdsk to see if it fixes your issue.
1. Login as administrator
2. Run CMD
3. Write the following without the quotations "chkdsk /F /R /X #:" (The #: is the flash drive's letter you wish to check). The others stand for the following F/ fixes errors on the disk, R/ locates bad sectors and recovers readable information and /X forces the volume to dismount first if necessary).
Hope that helps.
Boogieman_WD