logged off?

Every so often my PC has rebooted on its own.

Today Thur 14th Sept. it shut down(logged off ) then Auto started again right in the middle of typing a message on a utility website. Its not the first time its happened. its a pain had to start all over again on the utility website.
Any ideas on why it does it?

Windows 7.
Don't know anything else my son deals with the technical side and its jargon.
Many thanks