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If you mean you are the country refusing trade to others its simple.
Go to the diplomatic map, click on the country you refused trade to, then you should have an option to end the refused trade situation there.
You have an option to set a new trade refusal there too. But as long as you don't have a COT in your empire it's senseless to forbid trade to other countries ofcourse.
If other countries refuse your merchants to enter their COT it's a lot harder to get the refusal undone.
In principle you then have two options: either you go to war with that country, after signing a peace treaty the dtrade ban will be lifted.
Or you must invest in improving your relations with that country, and hope they will see the wisdom of lifting the ban in time.
I think you have gotten the random event that automatically make you refuse trade with all other countries for some years.
Note that this is a special circumstance so there is no bad effect from it. That is no free CB's or relational negative effects. You can't turn it down and why should you...? It is all for free.