Pictures would help. I had a house with well water and it was so cold condensation would form and that caused rot. I’d turn off the water if you have another toilet. Remove the loose and damaged drywall. Take photos then tape up a white tarp or opaque plastic sheet over the hole so it doesn’t look terrible and nothing falls into the kitchen. Post photos. Depending on what is revealed this might be a job you can fix for not a lot of money.