Car was on,ac was on,parked and i went out for checking air pressure of tyres,later i came in after 5min,and saw,car turned off and ac blowing hot air,
i tried starting it but
Cranking but no start,
checked fuel line,spark,starter,battery.
car doesnt start at all.
later changed the ecu with another,and car starts on one crank.
what may be problem,can the ecu get damaged this easy?

 

Was the ECU that did not succeed in starting the car the original ECU?

 

yes the original ecu wasnt able to start the car.

 

Possible the car overheated while it was idling and ECU could have gotten damaged. If the new ECU is starting the car and the old one isn't I think you found your issue. Make sure you inspect your cooling system as it must have an issue.

 

bt no overflow from the reservoir tank,if car overheats,some coolant should have come out from the overflow.

 

Not necessarily.
It's very unusual to lose an ECU. It's also very unusual to be able to switch the ECUs easily without reprogramming the immobilizer. Is the car stock?