I have a page on my site that has a block for HTML that I cannot delete!

While playing around with some coding, I pasted the html for the web version of a newsletter--you know the link typically on top of newsletters that says:

"Email not displaying correctly? View it in your browser"

NOW when attempting to delete the block with that html code the entire page is altered in such a way that the normal editing controls are not visible. FYI, the newsletter code is one that had a floating bar on the top of the page with 'social media' links: "like", "comment", "tweet", "subscribe", etc.

Is there something inherent in such code that overpowers the interface for the CMS?
How can I remove this code and get back to the display that I had previously OR add new different content?

Thanks