I was driving a went to the store and came back out and my car won't start I just replace the battery and the alternator!

If the Check Engine Light consistently comes on for 2 seconds and then goes out, when the ignition switch is turned to ON (Position II), then replacing the Ignition Control Module (ICM) and coil usually solves the problem. Be sure to apply silicone heat transfer grease to the back of the new ICM. This assumes you have a crank but won't start situation.