If the temperature is too low, enzyme molecules don’t have enough kinetic energy to collide and combine with other substrate molecules. So the enzyme activity/ reaction rate will be very low. The rate will rise again once the temperature increases to its optimum temperature.
But if the temperature is too high, enzyme will be denatured, which means the shape of the active site of the enzyme is changed. It can no long combine with other substrate molecules. The rate will drop significantly. Yet, lowering the temperature won’t help because denaturation is permanent and irreversible.