Right-click My Computer
and Left-click Manage.
Go to Disk Management
On the right at the bottom is a graphic interface of the drives on your computer.
Look for one that is the size of your flash drive and does not have a drive letter. You can Right-click the drive
its drive letter.
Go back to My Computer
and it should appear with the new drive letter.
I have seen this happen many times but that is no guarantee that is what is up with yours, but I hope this helps.