if your temp needle stays exactly where it is when the engine is off, then it might be
1. faulty coolant temp censor
2. disconnected wire or connector of the censor
3. faulty temp gauage
if it moves very slightly after 15 minutes of running
1. faulty coolant censor
2. faulty gauge
3. you must be in a very cold place and you radiator fan is direct.
for further info watch the embedded video
YouTube - Auto Repair: Coolant Temperature Sensors - Testing and Replacing