To go, retreat, etc. into your shell
Definition 1 of 1
Pro Tip 1/3Emotional Trigger
The phrase is used when someone is upset or embarrassed and becomes very quiet and less communicative. 
He's been quiet all day; I think the criticism made him retreat into his shell.
Pro Tip 2/3Temporary State
It implies a temporary state, suggesting the person may come out of their shell eventually. 
She's gone into her shell right now, but she'll be her usual self with some time.
Pro Tip 3/3Not Literal
This phrase is figurative and does not refer to physically hiding within a shell like a turtle might do. 
Don't worry, he hasn't literally disappeared, he's just gone into his shell for a bit.