Same thing happened with me in city IDSI. The over temp light came on and engine switched off on its own. I stopped on the road side, poured in water, luckily the radiator cap was still over the fuse box. Drove the car hassle free for 2 yrs or so after which I had to change the head gaskit. City Idsi engine is pretty robust.
Most probably reason is
Coolant leakage
Or
During service , some bolt wasn't tightened enough.
Try to get service done in your presence and don't fiddle unnecessary with engine and electronics.
Hope your car is now running fine
