The answer depends a lot on what goes wrong.

If you lose your break when performing Triumph and the wrong card is face-up at the end, there are various outs you can use; I, for example, would show the audience that the correct card had, in fact, disappeared from the deck (hence, it couldn't turn face-up), then have that card magically reappear in the deck, face-up as promised.

If you're doing an effect in which the central point is that you have only two cards in your hands, and you drop the cards (all sixteen of them), there's not much you can do other than gather up the evidence and try to show them some coin magic (if they're even still interested).

The overall answer is that you don't lose your cool, and that you recognize the difference between a situation from which you can recover and one from which you cannot.