Engine temp gauge not working

Check for error codes first by bridging the two ports in the diagnostic plug and checking the flashing check engine light.  It will tell you if there is a fault or a disconnected sender.
You can google to get the instructions for it.

Check the wire is connected to the gauge temp sender
Check the sender works (remove it and put it in water with a thermometer, raise the temperature and use a multimeter to check the values it gives)
If you've changed anything at the back of the gauge cluster, check those connections are all ok.
I guess the gauge itself could fail too but I'd say it's pretty unlikely.
This might help - http://mr2.neogruts.com/engine/engine.htm
 
They'd be my steps to test.

Thanks for all that info

I honestly don't remember seeing it working since I got the car, but the previous owner told me it was definately working since he never drove it until it had warned up to about the 2nd bar atleast. So it has stopped in the past 6 months. I don't know how I didn't notice but I think I first noticed it about 4 months ago

Nothing recently has been done to the car to make it stop.

I think I've got a used temperature gauge sender here if yours turns out to be faulty and you want a cheap fix.
 

Thank you I'll let you know if I need it

Connection or sensor