Why does my key lock the door but not open the door?

It depends on if it is a vehicle or house door. Many times the lock needs to be lubricated and it will start working properly. Some times the key is worn down and a 'code key' would need to be cut. If it is a Toyota it could be that the split wafers are bad. If it is a Honda sidewinder then it could also be a bad wafer. Need more information.