You’ve run out of ice or limes or another bar essential.
Again, stocking up beforehand will take care of most surprises. Run through the drinks you’re serving and all the ingredients you need for them. If you’re putting out alcohol and mixers, make a list of basic bar ingredients: lemons, limes, tonic water, seltzer water, ice, juices, and olives. Buy more alcohol than you need. Let’s be honest: If your guests don’t drink it, you eventually will. If you misjudged and run out of booze entirely, just keep guests going with any soft drink you have, water, or coffee or tea.