Welcome here on the forum.
Is that after starting the computer, right?
You can go on the session of Windows or Linux for example? With a Live CD, for example.
I need you to understand that the computer starts alone?
If you have set the parameters ( energy saving ) Enabled in your operating system (Linux or Windows) ...
Check Alimentation options in Config Pannel.
It is normal that if you touch your keyboard or mouse or even sometimes because of too much senstibilité mouse, it may happen that the computer restarts itself.
Do you WOL? Wake On LAN?
It will see the bios settings on the WOL, ERP, or PME Event Wake Up and choose option Disabled.
Check in your manual or .pdf file about your motherboard.
So if you have a questions, come here , do not take fear..
Sorry my english is not my natural language. I am a French person.
You read a french web page ? You understand it ?