I have a downstairs bathroom and since yesterday, I can't flush the toilet and the sewage is coming up through the bathtub drain. How do I fix it?

It means you have a blockage in the drian and You need to snake it from roof vent, good luck.

John

What is the roof vent??

Welcome navyg28

A roof vent is a pipe coming through roof and is usually in line with the fixtures below. This is used to add air to keep the traps at fixtures from having water pulled out.

This basic explanation.

Good luck

Chuck

Navyg28, you may have a clean-out near ground level that may be used to snake your drains. Good luck

Snaking from a second story roof vent in the winter can be dangerous.
I have a safer way. Remove the chrome overflow plate from the tub and send a snake down the opening,(see image) You will run into a return band,(the trap) about 18" down but once past that you should only put out another 14 feet. Good luck, tom