Well, plenty
You have either
1. Lost water
2. Got a thermostat stuck
3. Indicating fault.
Firstly establish if you have lost water.
If so, have you got an external leak?
check
Thermostat housing
Water pump
All hoses
Check for drips, check filler cap on expansion bottle
If no external leak but water loss, check oil filler cap for mayo (water loss past liners or, if you also have oil in the water, HGF)
If none of the above, stuck thermostat, could be it, start engine from cold and feel the hose to see if it gets hot, If not stuck thermostat.
If none of the above,you could perhaps have an air-lock, but then you would have been tinkering with the cooling system recently, and that you would know.
It just shows you how useless the temperature warning is, get another gauge
either install an analogue type as I did many moons ago, or get one of the excellent units of Hans.
And, with absolutely no modesty whatsoever!!!! install another durable thermostat (hint hint!!)
All the above assumes that you r car is V6 engine.
Otherwise all applies with exception of the thermostat housing!
If I have forgotten anything, it is just to see if the other big mouths on the forum are awake!!!