after purchasing our house, several years since, we notice that the ceilings are sagging. The joist are 24 inches. I dont know the thickness of the sheet rock. It appears that the sheet rock was not secured to the joist but on every other one allowing it to sag over the years. I can't push the ceiling up with my hands, it will not budge. Maybe a jack of some type would but I am afraid that the sheet rock will crack extensively and I dont know where I would get such a devise to force it up. then if I did, would it hold in place after screwing or nailing it. What is the best way to repair without tearing out the existing sheet rock?