Have you tried to enable debug mode, check your error logs?
What hosting plan do you have? How many users (active users) on your website?
Host on Arvixe.
That was only me Admin online.
I think it is because I opened some tags on same browser.
Because some code didn't finished, it looped and looped again.
That's why some code ate all my memory. But I was not able to release my hosting memory. so the bug destroyed my hosting system.
Have you tried to enable debug mode, check your error logs?
What hosting plan do you have?