Depending on your personality and performing character you could use comedy to cover this. If you have lost a card you could use two extra jokers or any other cards like two red aces and openly do the gag about having their card appear between. I usually will keep the jokers with my deck between the wrapper and the card box as they come in handy sometimes. What you do is put one card on top and one on bottom trapping not only their card, but the whole deck! A groan of comedy. Now there are some tricks, like Waltons Smiling Mule, which use this to neat effect if you want to learn that, but may not be the best way to recover in this instance. You could then just laugh it off and ask for their card, then go into something else with it that uses a single card as a start. You could also just say you were kidding and that now youll show them something extra mysterious and perform a totally new effect. If you go to use the aces and they had happened to pick one of those aces then maybe have a line about not being too bad, you narrowed it to two cards. Then come back with a killer effect that really wows them.
There are lots of controls and forces that may be easier in the beginning while you gain experience and comfort. Card College has some good ones to learn. One of the volumes also has a section on outs if Im remembering correctly but I cant look it up at this time.