Spackling paste or joint compound (drywall mud) Fine-grit Sandpaper or fine-grit sanding sponge Putty knife (aka joint knife, drywall knife) 6-inch drywall knife or 12-inch drywall taping knife (for ...
Something has shifted (like the house), come loose or has bent if it's not along a joint. Sheets of drywall shouldn't break like that. I'd take down the wall and redo it. It's going to break again.