First, have the phone number of at least two reliable vets, and confirm with them before a colic occurs that they will attend (if they are not otherwise engaged), no matter what time of day or night. I came across a veterinarian a few years ago who made it very clear she would only attend big money earners at hours suited to her, so that most definitely did not include any colic cases! So much for it being a vocation!!
Second, call the vet any time you suspect a colic. Describe the symptoms you are seeing, what the horse last ate, when they last ate, whether they have defecated or not and if yes what did their droppings look like. Have a thermometer on hand so you can take their temperature.
Thirdly, whilst calling the vet, remove all food and remove the horse from a stable if he or she is in one.
Signs of colic are;
Watching their flank
Pawing the ground
Kicking or biting at their belly
Curling the upper lip
Rolling, or trying to
Repeated lying down
Stretching as though to urinate, but not doing so
Sitting like a dog, haunches down, fores straight
Holding head in an unusual position
There are different types of colic and many reasons for a horse to colic - something for a whole other blog, or your vet to explain. The reason I am writing this is because again the myth of walking a colic'y horse occurred when a rescue horse a friend had recently taken in colic'd, and living in the middle of nowhere with the vet some 3 hours or more away, many people were quick to offer their advice to her. Walking a colic horse to exhaustion can do more harm than the initial colic. Walking them for 10 to 15 minutes, with a rest in between, and not forcing the horse to walk if they do want to stand still is perfectly ok. When you call your vet, if they tell you to walk the horse after hearing the symptoms, then do so. But do not listen to the diehards who believe a horse should be walked come what may.
The other myth in regard to colic is about stopping a horse from lying down and rolling. Consider that a healthy horse rolls often, and their intestines do not get twisted. I believe this myth came about in part due to horses in the confines of a stable getting caste or stuck half-way in a roll, due to lack of space.
I believe Doctor Ramey, The Royal Dick and many other institutions will confirm what I'm saying. Whatever your thoughts, please call a vet!