Everyone keeps talkin' fuel, no one's mentioned air.
You need air in, and a clear air path out.
Along the way, it needs to be compressed.

If it used to run, I doubt compression in all four chambers went incredibly bad at once.

Try with the air filter cover on. Can you feel any air going in?

Fuel
Air (inc. path for exhaust)
Compression
Spark

If you got all 4, it's a good question.