If you plug it in and it does not appear, it may be choosing the wrong drive letter.
If that drive letter is already in use it will not appear.
Open the start menu, right click on my
computer, then choose manage.
Select (on the left)
Disk Management.
If it appears in there.
Find the thumbdrive in the list and right-click and choose 'Assign a new drive letter' . Make it like M or somthing far down in the alphabet.
If that does not appear under the disk management then the disk might be physically damaged.
An urban myth (which does sometimes work) includes putting the drive in the freezer for a day or so.
Also remember it might be somthing as simple as a bad cable or port on the computer. Try to access it with a new cable from a diffrent computer.