The very first thing you want to do in a trailer rollover is to call 911 and have them dispatch the fire department AND a large animal vet. Developing a resource list for your dispatcher ahead of time is a service you'll never regret doing. Make sure you tell the dispatcher to inform the responders to approach with sirens and flashing lights OFF.
When approaching the trailer, size up the scene. What do you see that can injure you? How can you approach in a safer manner? If you're on a roadway, set out flares or get some help stopping traffic. Can you block the wheels or the hitch so the trailer doesn't shift? Can you tie thetrailer to a tree to keep it from moving? Did the trailer pull down electric wires? Is there a person/pet in the tow vehicle that needs help? Are there propane tanks/batteries that make the scene unsafe? Are there people in the trailer? Do you have a safe area around the trailer once the horse is out? Always walk around the trailer before approaching to make sure you're not missing something dangerous. Like the sign on the side of the trailer that says it contains circus lions!
Approach slowly and quietly, speaking quietly to the horse. Find the smallest possible opening, cover it with your jacket or whatever you have available, then open it very slowly. Let the horse get used to the movement. Usually, a downed horse will lie quietly figuring out he can't do anything about his situation. BUT the minute the smallest thing changes, he sees that as an opportunity to try again. Talk to him in a soft monotone. A singsong voice resembles a distress call. What do you see? Can you see the length of the horse? Look for limbs at odd angles or trapped, and rapid breathing which is an indicator of pain (and panic). Look for blood, and tears in the trailer walls that could be cutting the horse.
This is basically an assessment. You're not going to do anything at this point, just check him out. Do not attempt to get into the trailer with the horse. The safest course of action is to document what you can see from outside and wait for help. Getting a horse out of an overturned trailer is not a one-man job.
The exception would be in a stock trailer which is basically a big box. The trailer may be tipped over but is the horse standing? Assess the condition of the horse. If he was tied to the side he could be hanging by his halter or lying head down. Get a long rope on his halter then cut him loose. If he's not injured he will probably stand. Because the sides are slats you should be able to see him fairly easily. Even if it's daytime you may need to put on your headlight to see inside the trailer. Be sure the horse is comfortable with the movement of the light. Talk to him all the time you're near him and if you move away, start talking from a distance when you approach again. You may be able to stroke him through a small opening. Don't PAT him. That is more stimulating than soothing.
If he stands on the side of the trailer he may get his foot stuck in the slats. This is going to be a problem and you may not be able to resolve it. Responders may have something solid like a backboard or a piece of plywood they can slide under the slats to keep the horse from stepping through. When you call 911 you can relay this info so they're prepared.
What kind of exit door does the trailer have? A ramp will have to be securely tied back so it doesn't snap shut on someone. The "top" half of double doors (when the trailer is on its side) will also have to be tied back, and when it is, the horse will try to escape. The leadline on his head will help control him. Do you carry any hay or grass you can offer the horse to help calm him? Horses live by their stomachs and chewing helps to calm them. Just massaging the gums of a horse who's down (if its safe to do) can help calm him.
By assessing all this information and relaying it to the responders, you give them a head start in rescuing the horse. Remember: YOUR SAFETY IS NUMBER ONE! If you are injured trying to help the horse, you'll just be one more thing the rescuers will need to deal with when they arrive. Since people get helped first, this will also delay the rescue of the horse!