허탈해요 · 허탈했어요 · 허탈한 (+noun)
drained, deflated, dejected (after effort comes to nothing)
Example
열심히 준비한 행사가 취소되어서 정말 허탈했어요.
The event I'd worked so hard on was canceled, and I felt completely deflated.
Word parts
허 empty+탈 come off
Literally 'emptied out', as if all your strength has slipped away.