Sorry to hear, unfortunately a rogue or poor optimized query on a large table can bring a server down regardless of the resources. Is just a matter of time until the resources are depleted.
https://gfy.com/fucking-around-and-b...rver-load.html
Restart the vps and the site will come back online, maybe increase the time between cron jobs? However sooner or later you will have to investigate what is eating your ram, but i would advise you to start with the queries.
Curious, is your database MyIsam or InnoDb?