Question

if a potted plant is overwatered for a long time what will happen
 

Answer 1

what will happen ultimately is that all the air spaces in the soil will get filled with water and roots will not be able to respire. hence the plant will die.

Answer 2

FIRST THE PLANT WILL USE ALL THE TRAPPED AIR AND WHEN THE AIR WILL FINISH IT WILL RESPIRE ANAEROBICALLY PRODUCING ALCOHOL  AND LACTIC ACID WHICH WILL ULTIMATELY KILL THE PLANT