Hi Sharon Misoi,
Are you able to copy files out of the flash drive? I would recommend you to test the flash on another PC. Does the issue persist? Run the Command Prompt as an Administrator and type the following command:
Chkdsk F: /r
Replace the F with the drive letter of the flash drive. Wait for the process to end and try to access the data. I would recommend you to update your anti virus and to do a complete system check.
Hope this helps