It could happen to anyone. People bury a person alive to scare them or to get rid of them. In this situation, rely only on yourself.
Do not waste oxygen. In a classic coffin there’s only enough oxygen for about an hour, maybe two. Inhale deeply, exhale very slowly. Once inhaled - do not swallow, or you will start to hyperventilate. Do not light up lighters or matches, they will waste oxygen. Using a flashlight is allowed. Screaming increases anxiety, which causes increased heartbeat and therefore - waste of oxygen. So don’t scream.
Shake up the lid with your hands. In some cheap low-quality coffins you will be able to even make a hole (with an engagement ring or a belt buckle.)
Cross your arms over your chest, holding onto your shoulders with your hands, and pull the shirt off upward. Tie it in a knot above your head, like so: This will prevent you from suffocating when the dirt falls on your face.
Kick the lid with your legs. In some cheap coffins the lid is broken or damaged already after being buried, due to the weight of the ground above it.
As soon as the lid breaks, throw and move the dirt that falls through in the direction of your feet. When it takes up a lot of space, try pressing the ground to the sides of the coffin with your legs and feet. Move around a bit.
Whatever you do - your main goal is to sit up: dirt will fill up the empty space and move to your advantage, so no matter what - do not stop and try breathing steadily and calmly.
Get up. Remember: the dirt in the grave is very loose, so battling your way up will be easier than it seems. It’s the other way around during a rainy weather however, since water makes dirt heavy and sticky.
JUST TO PROVE TUMBLR HAS A SURVIVAL GUIDE FOR FUCKING EVERYTHING.
This will prevent you from suffocating when the dirt falls on your face. 
