I have a Ubuntu 16.04 (32bit) LEMP SSD, 512MB VPS (Digital Ocean). Its all the time memory usage bellow 60% and CPU usage also normal. After every 2 or 3 days this VPS down and can't even connect via SSH. Every time I have to restart the VPS through the VPS provider reboot option. But VPS provider dashboard display this VPS up (on). It hosted, 2 WordPress sites (not have more than 600 daily visitors).
How do I diagnosis the issues for this matter?