404 errors and security warnings

I am not a very tech savvy person, i manage a WordPress for my boss. Two days ago within 3 minutes i got 13 404 errors. I don’t know how to fix them, and now also when i load his site on my phone to check it, it says “security warning: there are problems with the security certificate for this site.” and that never happened before either. I don’t know what i did to make it go all wonky, and any help at all would be absolutely lovely.
the site is scottellwood.com

oh and a heads up, it monster mashes all the widgets on the right side of the page down at the bottom if your looking at the page from internet explorer, that’s a different unsolved issue.. help with that would also be lovely, but i would like to focus on why the site is not secure anymore.