If you haven't spent time meditating and practicing, chances are your spells aren't going to work. That would be like trying to power a flashlight with uncharged batteries (in some cases).
Especially if you are attempting spells that would require a large ammount of energy, or if you are attempting to do something that isn't possible, lack of experiance and lack of Preparation could cause failure.
Magic is something that needs reverence, faith, practice, perseverance and above all else it needs Dedication. You need to give yourself time to grow, and have the patience to see things through.