It is actually pretty easy for cigars to become dried out - either you found cigars that you forgot to put in your humidor - your friend brought back some cigars from his vacation and doesn't have a humidor, so by the time you got them, they were dried out - you had cigars in a humidor that you neglected to keep properly humidified... Either way, any cigar enthusiast will agree that there is nothing worse than to have perfectly good premium cigars get dried up and un-smokeable. However, all is not lost. In this case, take some time to examine the wrapper of the cigar(s) - if the wrapper is intact with no cracking or peeling, the cigar(s) is(are) salvageable; if the wrapper is cracked, peeling, or has begun to unravel, just throw it away, as it will never be smokeable.
Salvaging dried cigars is time-consuming and requires patience; however, it will pay off. You must understand that although you have revived them, they will fall short of their original quality and some flavors may be lost, but they will still be quite enjoyable. There are several methods and techniques that may be employed to rescue dried cigars and they all take several weeks to work. What is most important is to NEVER put dry cigars in a fully charged humidor right away - they require slow re-absorption of moisture. If dry cigars are exposed to too much humidity too soon, they will expand and the wrapper will crack, ruining the cigar. This occurs because filler tobacco absorbs moisture faster than wrapper tobacco, causing the filler to expand quicker and eventually split the binder and the wrapper. To assist with the process of reviving dry cigars, you will need a hygrometer to measure the humidity, help control the process, and know when the process is complete and the cigars are ready. Below are a few methods that may be considered.
Method #1 (for single cigars)
Place the dry cigars in a humidor that has a humidification device that hasn't been recharged lately. Here, the cigars may be left to absorb what humidity is left in the humidor (see...the reason a hygrometer is helpful). After about a week, the humidification device can be recharged and the cigars can be reintroduced to a fully humidified environment. If done properly, the cigars may be ready to smoke in about 4-6 weeks.
Method #2 (for single cigars) - Smokeasy's preferred method
Tools needed: Sealable "Tupperware"-like container or a Ziploc plastic bag; sponge (brand new unused); distilled water (or propylene glycol solution); hygrometer
Place dry cigars in the sealable container or plastic bag with the hygrometer and seal the container/bag. Leave for a few hours and check the humidity level on the hygrometer - providing you with a starting point for humidification. Add about a tablespoon of water or propylene glycol to the sponge and reseal the container/bag with the hygrometer. Now you are waiting to bring the humidity level up to 70%, so every 24 hours, open the container/bag and add another tablespoon of distilled water/propylene glycol to the sponge. Once the hygrometer reads 70%, that humidity level will need to be maintained between 65%-70% for about 2-3 weeks by adding the water/solution to the sponge as necessary. Every few days, gently turn the cigars over while in the container/bag. After a few weeks, the cigars would be revived and the process complete; however, this timing will depend on the condition of the cigars - bone-dry cigars may take up to a month or more.
Method #3 (for a box of cigars)
Find a damp location in your home, like a cellar or crawl space, where moisture gathers - place the open box of dried cigars in that location and keep it there for about a week (or longer if cigars are extra dry). During this time, rotate the cigars a quarter turn every few days. After about 1-2 weeks, transfer the cigars to a fully charged humidor (with humidity level at ~70%). Continue to rotate the cigars a quarter turn every few days. You may consider the process complete when every cigar has been turned the full way around at least 2 times (figure that will take about another 3 weeks of time).
Method #4 (for a box of cigars)
Turn the cigar box upside-down and moisten the bottom of the box, either by gently running water from the faucet over the bottom of the box, or using a wet sponge or paper towel to wipe down the bottom of the box. Put the box into a large Ziploc bag. The cigars should be ready after a couple of days. However, you may also wrap a damp paper towel around the box before placing in the bag and leave it for a week. Keep in mind that this method is a rather rushed procedure and will likely fail. Again, reviving cigars requires some diligence and cannot be rushed - the cigars did not dry out overnight, and they certainly won't be revived overnight.
Good luck to you - take your time with this, do it right, reap the benefits!