Why does the server stop responding remotely?

Question

linux

Hey!
I have a droplet (minimum, 512 RAM, one core) purchased from Digital Ocean with Ubuntu Server 16.04 LTS installed.
Apache, MariaDB, PHP7, sendmail, ssh server are installed on it. All new clothes are worth, nothing extra is not set.
The built-in firewall is enabled, connections via ssh, http / s are allowed
All this is running one site on Wordpress with rather low attendance (no more than 100 people per day).
The problem is the following - after about a day and a half of normal operation, the server stops responding. That is, I can’t access the site via http - the server is unavailable, and I can’t connect via ssh - port 22: Connection timed out
Although through the console in the control panel I go to the server normally, the RAM is 50% full, the processor is not loaded. After a reboot everything is fine.

Answer 1

DDoS? Or through an injection vulnerability, and your node is a member of the botnet?
Watch network activity/look for suspicious processes.

Answer 2

Similar situation, only another VPS, Ubuntu 14, Maria DB. The situation appeared precisely with the installation of Maria DB.
There was no time to reinstall to be 100% sure.
The server is a test one and has already experienced more than one reinstallation :) and there were no such lags before.