HELP! My header and footer have disappeared in the whole site and I don't know how to fix it :(

Hi there,
I'm hoping someone can help me, please.

Yesterday I was trying to build a landing page on my website. I duplicated it one of the "public pages" to do this thinking that I was using it as a "test" in the "not linked" section. 

I was following this tutorial (for the personal plan); https://insidethesquare.co/tiny/landing-page - . I added this code;

<style>.header, #footer-sections {display:none!important;}</style>

So far so good, until I realised that the code had applied to the whole website and now I can't see the header and footer in any of the live pages! :((((

In the tutorial it explains how to remove the code (simply by deleting it), but I don't know if it's because there is only one section in the page where I added the code, but I can't get the little "+" symbol to appear anywhere in that page, and therefore I cannot remove it. 

This is the page; https://estelallanospersonaltraining.com/workouts-2

Please help, I have no idea about web design, coding or anything. Someone built my website for me and since then I've been doing little simple changes.

Thank you in advance!