I'm assuming that some or all of the paper facing has come off. The paper facing gives drywall rigidity. Without it, drywall sheets will simply crumble.
What you need to do depends on the severity of the problem. Two types of damage are; a single layer of paper came off with a brown paper remaining or all of the paper is missing with the gypsum core exposed.
1) Brown Paper Remaining – The paper will need priming before any joint compound can be applied. This will help stabilize the surface and prevent most of the bubbling that can happen if joint compound is applied without priming. Use a generous coat of Kilz Original Oil Base. Allow it to dry overnight before proceeding.
Now the entire surface needs 1-2 coats of joint compound. This will get it back to a flat surface again. Any and all major cracks or exposed core needs taping for reinforcement.
2) Gypsum Core Exposed – Technically, any drywall without its paper should be replaced with new. This isn't always possible or practical.
One way to deal with this is to prime (same as above) and use wide fiberglass mesh embedded in joint compound to stabilize the damaged drywall. Wide sheets of fiberglass mesh can be found at stucco supply houses. This is used with EIFS (Synthetic Stucco) systems and are 4-5 feet wide. After embedding apply another coat of joint compound.
You will have to decide just how bad it is and what direction to take. Both of these techniques can be combined to suit your own unique situation.