Can you post a few photos of the area of the sagging ceiling panels? Whether the panels are the fabric headliner or vinyl elephant skin, they could have come loose when removing the upper cabinets or simply as a result of gravity and age. However, you can reattach them using a heavy duty spray glue. Don’t use a light
duty crafting type. Be sure to follow directions on the package and protect the surrounding areas. Ventilator recommended too.