Ideally, if you lose touch with the person you named as executor in your will, you'll revise the document to designate someone new. Oversights happen, however, so if you die before you have a chance to make the change, most states have contingency plans in place for appointing someone else.
The Executor becomes the Administrator
An executor is the individual you name in your will to handle the administration of your estate. If you die without a will, or if the court has to appoint someone else because no one can locate your executor, the person the court appoints is called an administrator. The role is usually identical to that of an executor -- just the name of the position is different.
Priority for Appointment
In some states, the primary beneficiary of your will has the first right to act as administrator if the named executor can't do the job. For example, in California, anyone to whom you've left at least half your estate can take an oath of office and assume administrative duties, provided he's a U.S. resident. If he declines to serve, he has the right to name someone else. In some other states, your surviving spouse may have the first right, followed by your children and then more distant kin.
Hedging Your Bets
Naming a contingent or alternate executor can prevent complications if your first choice should die, become ineligible to serve, refuse to serve, or disappear to parts unknown. The second person named would automatically take over.