I've never heard of 굿밤. Does this mean 'Good night'? This is not a 유행어.
'멘붕' must be the most trendy one, which means a 'mental frustration'.
'멘붕' is short form of '멘탈 붕괴'. '멘탈' is a Korean way to say English word 'mental' and '붕괴' is a Korean word for 'Break down'.
You can say 멘붕(이) 왔다. or 멘붕상태다. or 멘붕됐다.
You can say this when someone or something makes you very embarrased or confused, so you don't know what to do or you just can't think of anything to react.