runs but when stopped it will not start up again

Many things can cause a no start. As the check engine light is on, take it to a shop and have them read the code(s) stored. That would give you a direction to start from.

the problem is relatedd to sensors , i changed the crank shaft position sensor , and it was not the reason and i check the ignition control unit situated under the ignition key , it was faulty and the cause of the issue , but the major problem is unavailability pf this part .

my advise is optra is a good car but never try to buy it , if you have problems its parts are not available

its the common issue with all Chevrolet cars , so is it better to stay away from Chevrolet cars?

The Optra is not a real Chevrolet, it is a Daewoo Lacetti made in South Korea. GM had a deal with Daewoo at the time to sell re-badged cars in countries that GM did not sell a lot in.

As for parts availability, every brand out there has vehicles they do not support. Especially ones that were re-badged units.