Morris Minor Owners Club

I know nothing about car mechanics. I recently purchased a 67 moggie with an after market temperature gauge. I ran the car with a little less water in the rad than i should and the gauge went high. Now that i have put the coolant in, the gauge is still high. I assume it is stuck. Anyone know how to reset?

Partly depends if it's an electrical gauge or a bourdon tube gauge.
Is it a solid but bendy metal pipe to the gauge, or a singe electrical wire from the Sender.

Either should respond to a gentle tapping if it's just the needle stuck.
The electrical gauge can be more easily taken apart.

Do you mean that the gauge is stuck high, or it returns to low and then runs high after the engine has been running? Also, does it have a temperature scale or just a blue-black-red scale and the needle is in red? If not stuck and does have a temperature, at what temp is it running?