Check for water intrusion, particularly around AC units.

To repair detached ceiling vinyl, remove the white plastic strip to access the area underneath. Find a glue compatible with both vinyl and foam, such as 3M spray adhesive. After applying the glue, press the vinyl down and reinstall the plastic strip. If there is water damage, the plywood underlayment may be rotted or the foam degraded. New panels can be purchased, but they are expensive.