I had a friend's horse get into a bag of Omolene and eat half of it overnight( not my place, very " light" stall construction. When discovered he was longed for about an hour, was full of energy, Dan the entire hour, lol, then only a handful of his usual alfalfa and lord of straw, another longing session later in the day. He was fine.
So, no grain, for hay only grass hay, and yes, unless you have slowfeeders, in small portions throughout the day, and exercise.
Watch for typical colic signs, up and down repeatedly, looking and kicking at belly etc.
Call you vet, warning him that you might have an emergency, tell him what happened, so you can work out some kind of plan.
Feel their feet for heat, feel for the digital pulse.
Don't let them go without food, which could cause insulin spikes, which have to be avoided.