I made marshmallow fondant on Friday night and refrigerated overnight. Took it out Saturday and used it and just realized I put it back in the cupboard instead of the fridge. Will it still be okay to use it for a cake tomorrow, or do I just need to make a new batch.

No problem. If you had wrapped it well it should be fine. You may have to microwave it for a few seconds if it is too hard to work with.

HTH

I alway keep mine in the cupboard never in the fridge and its always fine.