2 Kings 20:1-11
Hezekiah was a good king, yet he was sick. Isaiah the prophet visited him and said "Set your house in order, you shall die and not live." Hezekiah's response was to pray. He reminded God that he had done that which is good in God's eyes and he cried out "with great weeping." Before Isaiah reached the middle of the city God spoke to him and said "return to Hezekiah, let him know I have heard his prayer and I will heal him and add fifteen years to his life."
Did God change his mind? Did the prayer of Hezekiah alter God's plan? From man's perspective the answer would be yes but from God's perspective the answer would be no. I'm convinced God new all along how Hezekiah would respond and that He would heal him and extend his life fifteen years. I know prayer is powerful but I'm convinced that prayer does not change God, prayer changes us and our circumstances.
Study these passages for yourself, ponder them and see what the Holy Spirit is saying to you - for prayer is indeed powerful.