Connection lost – Unable to update pages

Hi there, I am working with a client who I had originally created a site for about 6 months ago. Once the job finished I had not logged back in to update or service anything on the back end of the client’s site. However, I am now receiving this error message when I try to edit any existing pages:

Connection lost.Saving has been disabled until you’re reconnected. We’re backing up this post in your browser, just in case.

The steps I have currently taken (but have not fixed the issue)
1. update all plugins
2. turn off individual plugins to try to locate the issue
3. upgrade WordPress to 4.4.1

Can anyone help me with this issue please and thank you?