I have a 21hp B&S twin cylinder lawn mower that I have used without incident for 4 years. This past winter it sat idle in the shed until I was ready to use it in the spring. I filled it up with gas and, apparently not too carefully, checked the dipstick but stupidly enough did not put any new oil in it. I ran it for a while and then heard a loud bang, when the lawn mower died and would not restart. I believe I either snapped a rod or cracked the whole block. Does anyone know any more than I do about what I've done?

Not really without being able to inspect the unit. But it sounds as though you are on the right track. And not changing the old oil, if it was not contaminated did not cause this unless the oil level was too low.

Yep. Your on the right track. I did this same sort of thing as a pre-teen and my father said YOUR going to rebuild it. Never did it again. A puncture was visible on the block. Granted it was a 3 or 3.5 HP engine. It wasn't that hard.

You won't know what's wrong until you tear it down. You probably have a snapped rod. The piston may or may not have to be replaced. The rings likely will need replacing and the cylinder honed. You can likely get oversized rings if needed. The valves also could have gotten bent in the process.

Any news yet?