Lose your head
Definition 1 of 2
Pro Tip 1/3Pressure Trigger
This phrase often indicates someone couldn't cope with a stressful situation. 
During the debate, she lost her head and forgot all her points.
Pro Tip 2/3Stay Calm
It's used to advise someone to remain calm and not to overreact in a difficult situation. 
The key to solving this problem is to not lose your head.
Pro Tip 3/3Emergency Reactions
Commonly used to describe inappropriate or unhelpful behavior during emergencies. 
It's important not to lose your head during a crisis.