Take the wire off of the sending unit, on the side of the head(driver side, normally), and ground it to the engine somewhere. With the key on, the temp gauge should peg out. If it does not, there is a break in the wire from the sending unit, or a problem with the gauge, or cluster contacts.
If it does peg the gauge, the sending unit is not functioning. The connector may just be loose on the sender, also. Sometimes the terminal gets a bit spread out, and just doesn't make good contact.
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