Ruined, spoiled, or wasted (e.g., a plan, opportunity, mood). Often used as 'make something 台無し' or 'become 台無し'.
雨でせっかくのピクニックが台無しになった。
The picnic we were looking forward to was ruined by the rain.
彼の一言でパーティーの雰囲気が台無しになった。
His remark ruined the mood of the party.
時間がなくて準備が台無しになってしまった。
Because of lack of time, the preparations got messed up.