不再

no more / no longer

no; not so / (bound form) not; un-

again; once more; re- / second; another / then (after sth, and not until then) / no matter how ... (followed by an adjective or verb, and then (usually) 也[ye3] or 都[dou1] for emphasis)

Time that has passed will never come back. (idiom)

opportunity knocks but once (idiom)

Opportunity knocks but once. (idiom)

lit. one's youth will never return / to make the most of one's opportunities (idiom)