Make sure that God is the one who shut the door. If God really did shut the door then stop banging on it. Otherwise, Matt 7 "Ask and it will be given to you; seek and you will find; knock and the door will be opened to you.
I kept the other persons description because it just sounded so right: Everyone has a purpose. Maybe yours is to change the world. Maybe it's to change a persons world. Or helping someone who needs their world changed.
For the battle is not yours, but God's. - 2 Chronicles - Do you know the story behind this verse? It's one of THE most powerful in the Bible. Click through to read this powerful truth when you're facing a battle.