Source: https://www.youtube.com/watch?v=rafB8p9Eddo

Spell Casters lose their spell slots, they can cast their spells at will. Instead when they cast a spell, they make a Spell Ability Check DC = 10 + Spell Level.

If they pass the check they cast the spell as normal. If they fail the spell fizzles, it is not cast and you are unable to cast the spell until your next long rest.