After replacing the ECM, the car will not start?

When you say the car won't start do you mean the engine won't crank over (starter) or the engine turns over but does not run? Please check out this guide to help us get started on fixing the problem:

https://www.2carpros.com/articles/car-cranks-but-wont-start

Please go over this guide and report back.

It won't turn over.

Thank you, can I ask if the security light is flashing when you try to crank it over? Also was the ECM programmed before you installed it or how exactly did you replace the ECM?

https://www.2carpros.com/articles/how-to-reset-a-security-system

Please go over this guide and report back.