I stopped a freelance online teaching job in the winter of 2020 but then restarted it again in the summer of 2020.. how should I answer Did you stop working edu in 2020?

You should not check the box to say that you stopped the work during 2020.  

The question is essentially asking if you have stopped doing the self-employed work and you do not expect to continue it in the future. Stopping for a short period of time and then starting it up again during the same tax year would not be considered stopping the work.