When you tipped it to inspect the underside it may have been tipped wrong and flowed oil/fuel into cylinder. The first pull opened the valves and let the fluid flow from intake to cylinder. Fluids won't compress so the engine gets hydro-locked. Pull spark plug, ground plug wire to engine metal, aim plug hole in a safe direction and pull rope. If it spins and squirts fuel/oil all over the landscape all you have is a messy clean up to deal with.

Bob