2 years ago we put in a new toilet in our half bath and everything was fine until 3 days ago. It started to smell like sewer in there. Please help, can't take the smell. :eek:

It's possible for a wax seal to hold water back but allow sewer gas to escape.
If it's not rotting hair caught in the lavatory drain that's what I would check first. Good luck, Tom

Speedballs suggestion should be done first. If that does not fix the problem try inspecting the vent pipe. It should eventually terminate through the roof but might "T' with other vents prior to the roof.