Hi!

What do you mean by going down? Does it completely crash or would you still be able to SSH into it? Do you get any errors?

MySQL running out of memory and crashing is usually the case, try checking its error log for errors:

sudo tail -40 /var/log/mysql.err

P.S.: Ubuntu 13.10 has reached its EOL stage -- you are no longer receiving package updates or security updates. I recommend upgrading to Ubuntu 14.04 as soon as possible:

sudo do-release-upgrade

I dont know why I cant access my website. I know it doesnt wordpress fault. Like you said, maybe Mysql crashing all the time. But how can I know it's because of mysql?

And also "sudo tail -40 /var/log/mysql.err" command does not show me anything.