Something plastic stuck in the trap of the toilet
Disassemble and demount the toilet. Remember to plug the hole, since the toilet is the trap, and that's a direct path for sewer gas. Use something really big and obvious so you can't ignore it when you go to remount the toilet (people have forgotten to take out plugs, and regretted it) such as a few plastic grocery bags or rags wadded up inside another plastic groucery bag or garbage bag (you can turn that inside out when you remove it for tidy disposal.)

Take the toilet bowl outside and turn it upside down, backside up, etc while running water from a hose into whatever opening is currently highest, and shake it around a bit as well.

If you've somehow managed to get "whatever" so stuck that you really can't get it out, you're ready for that new toilet. In any case, you need a new wax ring seal. If you manage to dislodge "whatever" you're good to reinstall the original. 

Remember to remove what you have plugged the pipe with BEFORE you set the toilet in place on the new wax ring seal.