High cpu usage by 'root' in Ubuntu VPS server

My server was working fine until last week. But I don't know how someone have changed by ssh root password. After that when I checked the CPU usage, it always 160-190% on executing 'top' command. Some users told that sites are going down many times due to this..

I'm wondering how CPU usage becomes more than '100%'. Usually, my CPU usage was always under 10%, may go up to 20% on high traffic. It will be used be 'www-data'.

Below is the screenshot on ruing 'top' command in ssh putty.

Server configuration: OS: Ubuntu Web server: Nginx Ram: 2GB Control Panel: Ajenti