How can I see why a comment was hidden?

Every comment CommentGuard hides or deletes for you gets a label that shows why. For example:

A moderation filter, like "Negativity" or "Profanity": the label shows the filter that was triggered.
Hide all comments: the label shows the comment was hidden by this setting.
Mute: the label shows the comment was hidden because you muted the commenter.
Delete mode: comments that were deleted instead of hidden show the same label, so you also know why they were removed.

Where you see the label

Dashboard: on the comment itself. Hidden comments are also grayed out and have a crossed-out eyeball icon, see How to tell if a comment is hidden or visible?
Playground: at the bottom of the Moderation tab you can test any comment. The result shows the label of the filter that would be triggered.

Comment hidden by mistake?

Click the eyeball icon to make it visible again. Then check the setting on the label. For a filter like "Negativity" you can describe what your page is about in the advanced settings, so the filter understands the context better. You can also add keywords to your whitelist so comments with those words are never hidden. Test your change in the playground before you move on.

All filters and settings are explained in How to setup automated comment moderation?