call Board of Health or other local government housing inspection agency. This shows a clog somewhere down the main sewer pipe. that has to be found and corrected.
in most communities, if the clog is toward the house from the area roughly equal to the water meter, it is owner/landlord's problem. Below that, it is the government's problem.
either or both have the right, upon unclogging it and finding out that the clog was caused by inappropriate materials being flushed down the toilet, to give YOU the bill.
"inappropriate materials" includes EVERYTHING that is not human waste, toilet paper, items plainly marked "flushable", and/or the output of the garbage disposal.