（一天之间） in a single day; in a very short time
be destroyed in one day
毁于一旦
collapse overnight
溃于一旦
Once the time is ripe...
一旦时机成熟
If [When] one loses confidence, he can never expect to do his work well.
一旦失去信心， 就甭想做好工作。
Once the principal contradiction is grasped, all problems will be readily solved.
一旦抓住了主要矛盾， 一切问题就迎刃而解。