The float valve is not sealing the fuel .
The carburettor is overflowing through the vent
The EPA forced vents to be internal
So the fuel tank has drained and filled up your engine
1) remove the carb & clean it
2) check that the float valve is working
3) drain & replace the oil
4) run mower as normal then chage the oil * and filter if fitted again )
5) fit an inline fuel tap and use it every time you mow .