Hi what happens if a will has been left to a dead person and the dead persons wife and children are alive

A deceased person cannot be an executer of a will, for obvious reasons.
If there are no other parties named then it becomes intestate and is dealt with by the courts.

hi what happens if a will has been left to a dead person and the dead persons wife and children are alive

Depends on the wording of the probate statute in your jurisdiction, as well as upon the wording of the will. Either the bequest lapses, and the heirs of the person who predeceased the decedent (the "dead person") get nothing; or they get something.