If your DM decides that you’ve used up all of your spell slots for the day, you can regain them in a few different ways.
One way is to rest for a short amount of time. After resting, you will be able to cast any spell that you have prepared that day.
Another way to regain spell slots is to use a Spell Recharge item. When used, this item will restore one spell slot for each use.
The final way to regain spell slots is to use a Greater Spell Recharge item. When used, this item will restore all of your spell slots for the day.