Not know what hit you
Definition 1 of 1
Pro Tip 1/3Emotional Impact
This phrase often expresses a strong emotional response to something unexpected. 
After the surprise party, he didn't know what hit him.
Pro Tip 2/3Sudden Change
Use this when something changes so suddenly that someone can't comprehend it immediately. 
She won the lottery and truly didn’t know what hit her.
Pro Tip 3/3Metaphorical Use
It's often used metaphorically to describe an event that isn't a physical impact. 
The company merger happened overnight; the employees didn’t know what hit them.