Sounds like the sensor that watches the engine temp. and reports it to
the computer control unit is faulty or the wire has come off... Some
troubleshooting will be required, but the replacement sensor is cheap
and also reasonably easy to access. Start there.
I'm suspicious, since replacing the timing belt likely involved removal
of all sorts of sensor wires that were in the way. One got missed, or is
now damaged... or the sensor itself has just failed.
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