CPU Usage 100%. Now showing 'Page Not Found' can now no longer view any Post.

What to do here? In my cPanel it is now showing CPU Usage 100%. Update...the pages are viewable, I can no longer view any post. It is coming back with 'Page Not Found' when going to view.
I have de-activated BuddyPress and other plugins not needed and still Page Not Found.
I am running MultiSite and Prosite with no real heavy usage.
I was recently in touch with my hosting providers to inform them of the site running very slow, they replied I should upgrade to VPS.

Thoughts/Ideas/suggestions on what to do here?

Peter