Atv firing and getting gas but won't crank?

If an engine won't crank, it is a battery, starter, solenoid or wiring problem. Cranking means "engine turning over". If you mean that it cranks/turns over but won't start, then that is a different problem.

An engine needs air, compression, good fuel and good spark to run. It could be something simple like bad fuel or weak spark. It could be that the spark timing is off. It could also be low compression. (A worn cylinder or valves that are adjusted too tight or open/off their seats) It could be getting too much fuel or not enough fuel.

You haven't really given enough information to be more specific.