How do I fix this?

Hi,

That doesn't make sense. If the muffler were to fall off, the engine would still run. What exactly was done to the muffler?

In most cases, a crank no start condition is caused by either a fuel issue or ignition spark issue.

Here is what I need you to do. Try using starting fluid to see if it starts. If it does and then stalls, we know there is a fuel-related issue. If there is absolutely no change, check for spark to the plugs.

Here is a link showing how that is done:

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

Let me know exactly what was done to the exhaust and the results of the aforementioned test.

Take care,

Joe