Clean out the screwhead with a scratch awl, and, if it's catching a stud, use a cordless drill/driver to re-sink it. If it was a miss, back out the screw and use a new one to pin that section of drywall down at the nearest stud, just deep enough to hide under a thin layer of joint compound. Use the right fastener for the job: 1¼-inch drywall screws for ½-inch drywall, 1½-inch drywall screws for fire-rated or ⅝-inch drywall, and fine threads for steel framing. Nail pops are a different story. Sink a drywall screw into the stud several inches above and below the bump, then use a utility knife to cut around the loose nail. Use a cat's paw to remove the nail, levering against a putty knife to protect the wall. Then, for any of these fixes, apply several fresh coats of joint compound, sanding between coats, and prime and paint the spot.