Q: Engine heating issue.

While driving the ac cut off and after a while I noticed the engine light come up and disappeared. I noticed that the heat gauge has spiked so I stopped the car and opened the hood and found that the outlet valve from engine to radiator gave off the thermostat has popped out of its place, the coolant level was zero, then towed to the mechanic. The engine is running no difference found in the engine noise, it runssmooth. The mechanic found that the water pump is faulty and replaced it thermostat is also replaced along with it's housing and new coolant is added. Mechanic took the car for a drive and found that the engine heat indicator gauge increases above the middle point, please advice on what the issue would be.

My car is Chevy Optra Petrol 1.6ltrs My car has 29000 miles.
My car has a manual transmission.