Four test:

Are you banned from that community?
easy way to check: just try to access it while logged out. If you can, then you’ve been banned.

Did you block the community?
Easy way to check, go to the page, and check the sidebar.

Did either instance block the other?
Nope.

How to check:
https://lemmy.sdf.org/instances
https://lemmy.zip/instances

Are you accessing Lemmy through a phone app instead of a browser, and is that app blocking that community or instance?
Easy way to check, just use a browser.