Invoking the Prophet, calling upon Him and seeking Help from Him after His Death are all Forms of Major Shirk
Question :
Is calling upon the Prophet or invoking him in times of need and seeking his help from calamities and misfortunes, from near - I mean, from his grave - or far, an ugly Shirk, or not?
Answer:
Invoking the Prophet, calling upon him and seeking his help after his death in times of need and in search of relief from hardships is major Shirk, which removes a person from the fold of Islam, whether it is done near his grave or far from it, such as saying: "O Messenger of Allah! Cure me." Or: "return my lost property." etc.
Source:
The Permanent Committee
Fatawa Islamiyah, Vol. 1 Pages 278-279