SQL Server 2012 performance degradation
The first message regarding the shutdown means that someone or something has initiated a shutdown of the machine. Take a look at the Windows system logs in Event Viewer for more information. You should look at log events leading up to and just after the shutdown time to look for messages indicated why Windows shutdown.

Is this a relatively newly installed server? If so, have you activated Windows? If not and you've passed the activation grace period this is one potential reason for Windows Server automatically rebooting.