Could be a bunch of factors: Low oil level, or none. Crank-case flooded with gasoline. Lack of fuel shut-off valve, when stored. Checking of oil before starting, after several days of being parked. Carburetor float pierced, or leaking. Float seat damaged, allowing fuel to flow into crank-case, thereby diluting the oil in there.
Note: There are ways for any or all of those problems to be fixed, before the engine gets trashed by any or all of what i have just outlined, and several more that will be added by Bill K!
The part about checking the oil, after several days of parking the machine, is this: The correct way of checking the oil l;evel in the engine is to pull out the dip-stick, wipe off the oil that is on it, then pushing it back in as far as it goes, then taking it out again, and seeing how much oil is really in the engine! If gasoline has flowed into the engine block, either from a bad float or seat, it can and will give you an over-full reading, leading you to experiencing a blown engine. Or, if the engine uses/loses oil, from running, a leaking float will give you a false sense of security, and "BOOM" or "SKRITCHHH!" Ok, Bill--your turn!