If you’re talking about the main that’s in the ground, it probably has tree roots in it or is broken “clay tile” pipe with tree roots in it.
If it is still flowing, but slow, then buy copper sulfate & flush it down the toilet, per the directions.
It will kill the tree roots, and you may need 2-3 applications.
If this doesn’t help, then you’ll need to call a “roto rooter” type of service, that has a cutting blade on their “snake”.
I would get bids and not have it done on the weekend, as it will be at “emergency” rates.
Unfortunately we had to hire someone to resolve our issues. If you buy the sewer line insurance from your local water company, you will only need to pay the copay.
That is usually work that has to be done by a plumber. But if you have a sewer trap that you can access you can purchase the smaller snakes and try it that way first. But if it's roots I'm not sure those are powerful enough to do it.
We had a line that kept clogging and then ultimate the 50-yr old pipe made from compressed tar paper (who ever thought that would hold up?) finally collapsed and we had to trench and replace the pipe.
Your city water company usually takes care of it. Call them first.