One possible reason for this is a drive letter conflict. This will be the easiest to resolve and so it goes first

.
Shut down your PC. Plug in your flash drive and then boot.
You should now run this on the administrator account or your account should have administrator privileges.
Right click on "Computer" > Manage > Disk Management.
Right click on the Removable Drive Partition > Change Drive Letter and Paths > Change > assign any other free drive letter > OK
In effect you will be changing the drive letter to any other free drive letter. Make sure you map a drive letter that is not in use, preferably x,y,z. If that works you can still change it to another drive letter easily.
If that does not work check whether the manufacturer of your Flash drive has any recovery program available. Make sure that such a recovery program does not format your drive. Since you did not give details of your flash drive like manufacturer, Model etc., you have to find it out yourself. (" My Flash drive/ device" and I do not have clairvoyant power or extrasensory perception

)