Whether you want to or not, you are going to have to send your child with formula. Breat milk keeps unrefridgerated for a long time, but not that long-- I believe the max is 8-12 hours. Look at it like this-- would YOU want to drink a glass of milk that had been sitting out in the same conditions after that long? The answer is irrefutabley, YES, it's bad now. DO NOT feed it to your child. One day of formula in the whole scheme/scope of your child's infancy is not such a big deal.
I have BF for over 12 months now. At least 80% of my son's liquids are "mommy's milk," but he has had to take the occasional glass of formula when mommy is away on business or simply didn't get enough on a specific day. It's more about what you do REGULARLY. So you made a minor mistake, and I bet you'll be that much more cognizant of taking care of your milk now-- am I right? So it's no big deal, send baby with formula, and then tomorrow baby will have milk again. Give yourself permission to relax a little.