Debris fell into engine block

So my problem is that after changing thermostat I took off the filler cap for my coolant that is connected straight to engine block not the radiator and bottom piece of the cap had deteriorated subsequently a large piece of the plastic about the size of a garden hose o-ring fell down into the engine block. I'm concerned flushing my system may not get it out. Any tips or tricks anybody has would be awesome, ASAP. Thanks