멘붕. 멘탈붕괴

'멘탈붕괴, Mental Bung Gwoe, 崩壞,  Mental breakdown, collapse

'멘탈붕괴' is shortened to '멘붕'

'멘붕 Men Bung, Mental breakdown, collapse' 

This is an expression used when the thoughts that were barely maintained are suddenly shattered and you don't know how to deal with it.