How can I fix a possible mistake in modifying the default page template?

[Iwannaknow]

Hello! I created a new page today (Pages -> New) and after updating it, I realized that my About Us page was pulling in the same content (all the previous content disappeared) (as shown on the new page).

I was seeing this note 'Default theme/page' at the top of my Theme view/where the content on each page is being updated so I think somehow my Default page was modified and this is why.

I solved the issue so the About Us page displays the right info now (I chose the About Us page template) but I'd like to know if I modified the Default page template by mistake. If yes, how can I reverse it so that all future pages will display the right info?