In the world of Harry Potter, Wizards are able to cast spells by drawing upon a reservoir of magical energy known as a spell slot. When a spell is cast, the wizard expends a certain number of spell slots, which are replenished as the wizard uses them.
When a wizard reaches the end of their spell slots, they must either find another source of magical energy or cast a spell that requires no magic at all in order to complete it. If a wizard casts a spell that requires magic but doesn’t have any remaining spell slots, they will suffer a severe penalty to their attack and damage rolls.
In order to regain lost spell slots, wizards must find an appropriate source of magical energy. This could be from absorbing the energy of another wizard during a duel or from using magical objects such as wands.
Once an adequate amount of magical energy has been found, the wizard can begin regaining their lost spell slots. .
This process is not easy and can take many weeks or even months depending on the level of difficulty involved with the particular spell. Once all of the missing spell slots have been regained, the wizard is once again fully capable of casting powerful spells.