Page not found error on website

I have this page not found error on my website. I have not changed my website for months now and suddenly for no reason this error appeared

I have been to the server host several times and they said it is to do with the theme and not them. I have read forums and they mentioned something about someone hacking the theme and need to delete some code (which I could not find)

The error is below:

The page you tried to access does not exist on this server. This page may not exist due to the following reasons:

1.You are the owner of this web site and you have not uploaded (or incorrectly uploaded) your web site. For information on uploading your web site using FTP client software or web design software, click here for FTP Upload Information. 2.The URL that you have entered in your browser is incorrect. Please re-enter the URL and try again. 3.The Link that you clicked on incorrectly points to this page. Please contact the owner of this web site to inform them of this situation.

I have tried to disable all the plugins but it has not worked either

Any help would be appreciated. Website is https://www.business-at.com