The prophets and their successors are infallible and they are the most humble servants of Allah (SWT). Their life is not limited to this world but continues as we read in Quran ( and don't think that those who were martyred in the way of Allah are dead, nay, they are alive with their Lord getting sustenance. (Sura 3, Verse 169). Prophets and their successors are higher in degree than the martyrs, so, they are alive with Allah.
Every Muslim salutes the Prophet in every Salaah ( ASSALAMU ALAYKA AYYUHAN NABIYYU WA RAHMATULLAH WA BARAKATUH) which means that the Prophet is alive.
We have many authentic Hadeeths that the Prophets are alive in their graves. ( الانبياء أحياء في قبورهم يصلون)
Seeking help from them after their demise is just like seeking help from them before their demise, as Allah (SWT) grants them the ability to help sincere believers by His permission.