In the absence of any helpful information from the OP, I am going to do a WAG that the timing belt (?) was not installed correctly.
However, that is just a guess from afar.
You can help us to give better guesses by telling us exactly what is happening when the engine doesn’t start. You can also help us to help you by telling us whether you replaced the timing belt (?) proactively, or whether the old belt snapped.
Is the engine cranking when you turn the key, or do you only hear a clicking noise, or do you hear no noises at all?
If the engine is cranking…Is it cranking at the normal speed, or is it cranking faster than usual?
Have you verified whether there is compression in the cylinders?
Have you verified whether there is fuel going to the engine/
Is there spark?