I am assuming that the skimmer was doing ok, making a modest amount of skimmate, and then all of a sudden created a lot of foam.
Here are some things to check.
Did you use any additives or meds in the tank?
Did you add any new livestock? Some LFSs use products such as Stress Coat when they bag fish, and this causes a skimmer to foam.
Did you get anything in the tank to contaminate it? Did anything die?
Did you change any filter media, with out washing it first? Sometimes the various filter materials contain things that make the skimmer foam.
If anything broken on the skimmer or pump?
That should get you started.
If the skimmer is not broken, it is usually best to let the skimmer remove whatever is causing the foaming. You may need to back down the pump's flow a little so the foam gets concentrated.