Inside the garage the ceiling sheetrock appears fine no sagging but in the attic, on one of the pieces the sheetrock has dropped for the joist and exposed 2/3's of about 6 nails. There is no popped nail heads on the sheetrock inside the garage. Can I just brace the sheetrock and screw the sheetrock to the joist? Do I need to remove the sheetrock section and attach a new sheet?
If the ceiling is flat without any swails or bulges then it is quite likely that the one joist is out of alignment with the others. It may have bowed up or was never aligned properly in the first place.
If the ceiling sheetrock was in stress being installed onto that particular joist it is entirely possible for the nails or screws to pull through the material from the back without disturbing the surface on the bottom. Usually in the case of popping nails the cause is that the nail pushes out toward the finished surface.
If the ceiling surface is flat, which you can tell by pushing the edge of a long straight board up against the ceiling from below, you would not want to try reattaching the sheet to the same joist. It would be more appropriate to fasten a board against the side of that one joist that projects down to the upper surface of the sheetrock. Then secure the sheetrock by screwing into that new board.
On the other hand if the ceiling has bulges and swails then you will have to re-evaluate and take appropriate steps. This could span the range of possibilities of accepting the way it is to complete removal of all or parts of the ceiling and shimming joists and replacing the sheetrock.