If the water gets into the cylinder head, it could create rust on the valves, cylinder liner(s), piston(s) and piston rings, causing the engine to "lock up".
Your mower has an electric PTO clutch that is worn out. It clutch stops working when it gets hot. Replace the PTO clutch and you should be fine. Expect to pay about $150 for the part.
the motor gets lubricated.
im asking the question, what happen when water gets in brain
The water gets cooler
Depends on where, and how much. There will be a little there due to condensation, which doesn't really affect things. But if you get a lot of it, it can destroy the oils in the pan, and with that the lubricating system. Or if it gets in the cylinders you can get something called "hydro lock" which can destroy the engine. Or get it in the electrics, and the engine will most likely not run.
Push mowers have kill switches that are on the handle and kill the engine if you let go and riding mowers sometimes have kill switches under the seat to kill it if the rider gets off the mower.
it gets cooler
It gets frozen
When tap water gets contaminated there is high risk for diseases to spread.
its gets cold Actually the ice gets warmer. Its the water that gets cold.
it gets cold and freezes
It gets wet.