The 18th Amendment prohibited alcoholic beverages; the 21st repealed the 18th. If the 21st Amendment were repealed, would that "un-repeal" the 18th Amendment and reinstate Prohibition?
Since one of the things that the 21st amendment does is repeal that Amendment, the 18th Amendment would again be a valid part of the Constitution (until SCOTUS says otherwise). Also note that prior to repeal of the 18th Amendment, the restrictions were lightened e.g. via the Cullen-Harrison Act. In the original instance, enabling legislation had to be passed – the National Prohibition Act. That act was not formally repealed or amended, it just fell to desuetude. Congress could re-pass enabling legislation like it did the first time. Or, the executive branch could conclude that the National Prohibition Act is good law, it would be enforced, there would be a lawsuit, and SCOTUS would decide.