You probably have too little magica. The next time you level up, boost inteligence. You get more magica and you will be able to cast your spells.
If you don't have too little, either get training from someone in altercation, or a cheaper way would be to get a lock spell at level 10. The door will be locked at 10 and you could either unlock it with a lock pick or with a spell. (But that is your problem isn't it
)