fall flat on your face 摔趴下；（颜面丢尽地）彻底失败；〈谚语〉惨败

【例句】
His next television venture fell flat on its face.
他的下一个电视项目丢人现眼，彻底失败了。

I'll help you out. I won't let you fall flat on your face.
我会帮助你，不会让你失败的。