Editing pages – Error 404

Hi all,
So recently, I’ve been tryng to make an update to one of the pages on my wordpress website. However, when I select “Preview Page” or “Update”, the page I am trying to edit, loads, but then redirects to an “Error 404 – Page not found” screen. I am not the site administrator, but what I cannot understand is why I’m unable to remove something from said page, when a couple of weeks ago I was able to “Add and Element” to the page without any trouble.
Keeping in mind that I’m not 100% clued-up in terms of all this technical stuff, but if anyone possibly has a solution, it would be greatly appreciated!

Thanks in advance,
xx