Well in this crazy world, sometimes people don't show up. They don't show up when they're scheduled, or at all! It is amazing how many people don't even show up for the interview they scheduled with you yesterday- without the courtesy of a phone call, e-mail or even text message!
So, what do you do?
The show must go on, as they say. The customer doesn't care who is or isn't there, they want and expect service. And you should make sure they get it. Even if it means you are picking up tables, answering phones, or cleaning rooms - - whatever the task is that needs to be done!
Of course you should call other crew members to cover the shift, but be prepared, people make plans. Ultimately it is your responsibility as the manager to ensure that your guests are taken care of. You are the one they will complain to and you are the one that will have to comp the check if the server never gets back to the guest to find out the meat was overcooked, and his kids never got their fries.
Be out there- if you are short staffed, be on the floor talking with your guests, letting them know that you are keeping an eye on things and your crew is doing the best they can under the circumstances. You will be surprised at how the public reacts to someone in distress. More often than not, when told in advance of the issue, the guests will be patient and kind. And, if they aren't - well there isn't much you can do about it under the circumstances, so stay positive and motivating to your crew that IS there. Let them know you are a team player, and on their side.
That will get you far!