If cigars are very dry, they will be difficult to revive satisfactorily. But, essentially, if moisture can escape from a cigar, it can also be replaced. One of the simplest methods, which usually works, is to put the open box of cigars in a large plastic bag, which is partially, but not completely, closed (it is essential to have a little air flow). You should also put a glass of water or a moist sponge in the bag. Rotate the cigars every few days, remembering also to bring cigars from the bottom of the box to the top, and within three weeks or so the cigars should return to smokable condition. It is very much a matter of trial and error, and means that you have to keep a careful eye on things. However, having been dry in the first place, they will have lost much of their bouquet and won't compare to a well-kept cigar. In any event, cigars lose moisture slowly and need to regain it equally as slowly. You need patience: attempting drastic measures will only ruin the cigars for good.
Another simple way of reviving a box of cigars is to turn it upside-down, and put it under a gently running faucet. Be careful: the bottom of the box should be moistened by the water, but no more. You could use a sponge as an alternative method of dampening the bottom of the box. Shake off excess water and put the box in an airtight bag. The cigars should be in good shape after a couple of days.
Some major cigar stores, particularly if you are a regular customer, will revive a box of cigars for you in their humidified room (it takes around a month) as a favor. The charming and knowledgeable Edward Sahakian of the Davidoff shop in London will even provide this service for people who aren't regular customers at no cost. "The pleasure of doing it is sufficient for both and myself," he says.
Top cigar stores will also store cigars in their humidors for regular clients.