Nightmare spell, open spell slots

[into the aether]

If a wizard who was subject to Nightmare had open spell slots from the previous day, could they still spend the time and prepare spells in them? Nightmare says arcane casters cannot regain arcane spells for the next 24 hours.

[Rajnish Umbra, Shadow Caller]

I'd say yes. If they could sit down and prepare spells in those slots the evening before, then they're not losing that ability overnight.