If the Drywall is only cracked, but not pushed down at the nail/screw attachements, go back into the attic,,, Take plywood but use it for attic flooring, at least walkways. Above the area you stepped into secure a 2 x 4,,, or two, between the studs/rafters, then from the bath ceiling tap nails, or screw drywall screws up into those 2 x 4 brace pieces, mud, sand, paint, and be careful next time,,,smiles.

Rev. Steven
References :
40 plus years as a contractor

First of all, preserve a section of the damaged material that has paint showing on it.

Look at the area to be painted once the damage is repaired. Despite what one respondent said, you don’t need to do the entire room. Just paint from the damaged corner back to the next corner, in both directions. I’m betting a quart will do the job; you can even water the paint down a bit if you’re running low.

Take your paint sample to the store and get a quart of that same color -they can get very close to the original with a special machine that reads color.

While you are there, pick up:

1. Light weight spackling compound, two small containers should do the job. Red Devil makes it. You know you’ve got the right stuff when you pick the container off the shelf and it feels empty -it is that light!

2. A putty or spackling kinfe/scraper -if you don’t have one already.

3. A cheap paint brush and/or roller kit if you don’t already have them.

4. A sanding sponge block -it looks like, and feels like, a small sponge. Even though it feels soft, it does have a tough grit and will do the job. Because it is a block, it will maintain contact evenly with the wall as you use it.

Now back home:

When you open the container of spackling compound, mix the top 1/2" or so with a putty knife, stirring and pushing it around until its like crisco. If you don’t, it will break apart when you try to apply it. If it helps, put a big dab on a flat surface, like a cookie sheet and mess it around until it will stick to a putty knife without breaking up. Play around with it a bit so you see how it "works." Do this now and then as you use the spackle so it stay creamy.

Clean up the damaged area, removing bits of plaster and shreds of the paper so all the surfaces are solid. Then use the putty knife to apply the spackle, but only fill the damaged area about half way to the surface and let dry overnight. Next day, fill to to be even with the wall. Because you’re working on an outside corner, you should be able to use the long side of it to smooth the surface. Don’t over-work it; once it is "about" right, let it dry overnight again, then sand it smooth next day, using your block in a circular motion.

Now repaint, and you’re done!
References :

Cut another peice of plywood to fit inbetween the rafters and long enough to cover the length of the crack from above (in the attic). Glue this with liquid nails or another good construction glue to the back of the cracked wallboard. You should also glue along the sides where it hits the rafters. You can brace this with additional 2 x 4’s glued on top and nailed into the rafters on either side as well.

Allow this to dry before removing the "T". Then fill the remaining cracks with joint compound and paint. You can also put a few screws up through the sheetrock into the plywood above if you think it needs it.
References :
Believe it or not it just came to my mind.