Is gauge registering nothing at all, or just rising a little?
This could be serious, so needs diagnosing and sorting asap.
Options:
Gauge gone pop. Inconvenient. And rare.
Sensor given up. Inconvenient. The fuel injection I think uses a second sensor, but if reliant on the one, it will now overfuel, thinking engine is cold. So needs sorting.
Thermostat stuck open. Engine will run cool, and ECU will overfuel. Needs sorting.
Wiring or connectors broken, damaged, disconnected. Usually easy to sort.
Coolant leak. Serious.
Don't just rely on there being coolant in the reservoir, leaks can lose coolant and the reservoir selfishly holds onto its contents. There are two bleed points, top heater hose near bulkhead, and top of radiator. Open them gently to ensure full coolant.
In normal warm-up, thermostat housing gets warm, but outlet to hose warms gently just through conduction. When thermostat opens, hose gets hot quite suddenly, so easy to check, just don't get fingers or clothing near the fan or any other moving parts.
If hose gets warm all along, stat is open.
Coolant loss from water pump is often invisible, as they tend to leak only when running, so the coolant is warm and evaporates, so no puddles.
Leak at radiator tanks tends to be gentle, so look for staining the colour of the coolant, should be pink.
Leak at heater matrix inside car hides well, so kick panels may need removing for a check. Causes a sweet sickly smell inside, and can cause condensation problems inside.
All fixable if caught early.
