A few weeks ago, I signed into my account and noticed a comment on a member of my family tree. I was somewhat distracted and accidently clicked out. When I went back the comment was gone and I have no idea how to get it back or where to find it.
Do you remember the profile on which you saw the comment? If you do, check the changelog of that PID. It's possible the person who made the comment has deleted it, but there should still be a breadcrumb in the changelog of the profile.
For example - using beta.familysearch - I put a note on the profile of a deceased cousin and then deleted it. You can see the breadcrumb in the changelog: