Thread ordering (General)
What your describing is the 'Thread Order' setting, which is something you may have accidentally toggled without realizing it.
Toggling this setting not only changed the default in your user profile, it also sets a local cookie with the setting, so when toggled it will still appear that way whether you are logged in or not.
There is no easy way for the default view to have changed accidentally, so the problem is almost certainly just your local setting.
If you want to verify what the default view of your site actually is, just open your site in a private or incognito browser window. Whatever view you see, that's the default.
Assuming it's just your view, just click the order option again to toggle back to the default view.