Go off the deep end
Definition 1 of 2
Pro Tip 1/3Emotional Trigger
Used when an event causes a strong emotional response leading to an extreme reaction. 
He went off the deep end after losing the championship game.
Pro Tip 2/3Not Literal
It's a metaphor for losing control, not someone swimming in water. 
When the airline lost her luggage, she went off the deep end.
Pro Tip 3/3Informal Use
Most suitable for casual conversations, not formal situations. 
At the party, Dave went off the deep end after too much to drink.