A. It is something that I accidentally do from time to time. There is an Ignore button at the top of the page when viewing a thread. You must have clicked that by mistake. After that it becomes an Unignore button which you can click immediately if you notice.
B. If, like you, you didn't do it at the time, it is a little trickier to restore the status quo because you can't so easily get in to click the button again!
Click on your avatar at the top of the page. There is an option labelled Ignoring. Click that. Click Threads. Click the name of the thread in question. Finally. Click Unignore Thread.