What to do if your car won't stay started?

Ignition is governed by 4 things

1. Air
2. fuel
3. spark
4. mixture

Check each of these exist in the cylinder. For instace check the air filter isn't clogged. That the fuel pump is working, that the spark at the plug exists, and the ECU/carberettors are working.

Have your mechanic check it out. It could be anything from a bad alternator to a bad fluid sensor. The only way to really know is to have it properly tested by someone qualified.