Troubleshoot reasons the engine won’t start

That' is very little information.

Does the engine turn over at all?

What is the make, model and year of the vehicle?

What is the motor / what displacement is the motor?

It broke down somewhere or you parked it and it wont start?

Assuming the engine turns over. You don'thave air, fuel or spark. Possible timing belt/ chain failure. Again, to little info.

Assuming the engine will not turn over. You have a faulty electrical connection, faulty vehicle control module, seized motor. Again, not enough info.