In the name of Allah the Most Beneficent and Merciful.
Just as it is possible for a living person to be given the power by Allah to help, the dead can also do the same as long as they are those who have not been subject to the wrath of Allah and are close to Allah. If a person believes that a living person has his own power and is not from Allah then it is as much Shirk as it is to believe that he can have the power after his death. Beside the Prophets, saints and martyrs taste death and are then given life in their graves. In the Quran we have been instructed not to call the martyrs dead “And do not speak of those who are slain in the way of Allah as dead, nay they are but alive, only you cannot perceive it” (2:154)
I suggest you read through the beliefs section on the website titled `Tawassul`.
Of course Allah knows best
[Answer provided by: Muhammad Salim Ghisa]