try a compression test to make sure there’s enough pressure in each cylinder there for it to start, your only need air, fuel spark and compression in the cylinders for it to start so one of them is being compromised. another thing to check is the timing adjusted properly as this will cause it to have trouble starting or not start at all. and would also cause it to randomly stall too, has it had any other symptoms building up to the car not starting that may have been overlooked like lower mpg, struggling to start, loss of power etc..?