If there are no altar servers, the priest simply makes appropriate modifications / simplifications. This most likely would mean - no procession - Altar already set - Missal next to the priest’s chair etc.
That said - the only times that I can think of where there might be NO servers available would be a weekday mass where only a very few people are present. Pretty much any Sunday Mass is going to be well enough attended that there will be someone available and willing to serve…
When I as a child and my family attended early mass on Sunday…If it started getting late and there was no activity - no candles lit…one or more of us boys would head up to see what was up and if no one was there…we “suited up”.