Use a toilet auger (not a regular drain snake). Insert it past the P-trap and crank gently. If you feel resistance, you might have found a local clog.
Before you call a plumber (and drop $300+), try these 4 steps: how to unclog toilet main line
Look for a white PVC or cast-iron pipe cap with a square knob outside your house, near the bathroom, or in the basement. Pro tip: open it slowly, and have a bucket ready. 🪣 Use a toilet auger (not a regular drain snake)
