Toilet clogging and overflowing every time i try plunging it

My toilet has been clogging for a while now but usually a good plunge takes care of it. I've noticed also that the chain has come unhooked from the flusher handle a few times in the last week and I think it's done that to my granddaughter as well & I've told her to tell me if there's any problems but I think she may have messed with that & maybe other parts in the tank because now the tank is super full of stinky sewage smelling water like my toilet and every time Ive tried to plunge it over the weekend trying to fix it it just overflows. I was trying dish soap and vinegar mixes ect to help unclog it but they didn't help & plunging is still making it overflow. I turned the water off at the line to toilet yesterday so I don't know what could be going on help!!?!?? Please & thank you in advance for any suggestions