"I needed to think quickly. I had already used up my most powerful spells I had prepared. What remained were minor cantrips and illusions. My mind raced to find which spell I had empowered in my mind that could help us now against this challenge. If only I had time, to sleep and rest, I could re-prepare my most devestating magic."
That doesn't even involve talking about game mechanics, but clearly indicates that magic users must prepare spells in advance in order to cast them, and that certain spells had already been used and couldn't be used again for a while.