I did the quest where you have to defend whiterun from the stormcloaks and now after that quest is done my house is lock. The key to the house is inside of it.

try to talk to steward

Are there still barricades and other left-overs from the battle around the city? If so, then all buildings will probably be locked. Leave the city, travel to a different location, perhaps do a small quest, come back to Whiterun, try again.