How to undo "Switch back to old editor"?

Unfortunately I clicked the “switch to the old editor” and it’s redirecting me to wp-admin panel of the site.

You can access the new editor here:
https://wordpress.com/post/

You can then pick the site where you want to post from the dropdown at the top of the page.

I hope this helps.

Doesn’t work, when I click on https://wordpress.com/post/ it redirects me to https://mysite.com/wp-admin/post-new.php ??

That will indeed happen if your site runs an outdated version of Jetpack, if Jetpack is not properly connected to WordPress.com, of if the JSON API module is disabled under Jetpack > Settings in your dashboard.

Let me know if that helps.

It’s working! I guess JSON API was the problem, thank you Jeremy!!