@mustang69rox , Lisa, If the toilet is backing up even though you had not flushed it, say you came home and found it had overflowed and know no one had use it, then the blockage will be further up the sewer line and sounds like it is being pushed back up the toilet. There are what they call clean out's at the base of most main stacks/sewer lines and also clean outs/removable caps at the sewer line main where it enters the home sticking out of the concrete floor normally. Some municipalities offer a sewer snaking service, where the towns utility service will come in and use these clean outs to snake out tree roots and other blockages. If the city does not offer the service or you have a septic system, the choices will be to have a plumber or handy man come in and snake out the sewer line or go to the rental center and rent the drain snake yourself(be sure you are instructed by rental on how to use), I find it is better to have 2 people to help load and unload the larger machine 75-150 feet coil machine. Below are some video links that may help. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
https://www.youtube.com/watch?v=_QQ7YxatBkY
https://www.youtube.com/watch?v=90mSI_Ukf8s
https://www.youtube.com/watch?v=p82Syagsyns