First thing I'd guess would be hydro lock. Remove plug and see if it cranks. CAREFUL, make sure key is off... if it was a leaky carb filling cylinder, and you crank the gas out the plug hole and generate a spark, big explosion or fire.
Second thought is compression release not working.
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Don '05 KLR 250; '09 Suzuki TU250x; '70 Honda Trail 90