the plasterboard going downstairs has started to crack and sag down in few places near the top is it possible to screw it back up?

Yes it is,providing it is actually plasterboard.if its not,it will have to come down.

The simple answer is yes.If the plasterboard has popped away from the fixings it can be screwed back in to position, providing there is nothing to stop it going back. Hope this helps. J Chappell Joinery

Yes if there is no obstruction underneath but you should use 3 to 4 screws abour two inches apart and pull them up bit by bit until the board has gone back into place,if you use this method you reduce the risk of the screws bieng pulled through the plasterboard.