Simply within SSMS right click on the instance and hit STOP and
To start an instance if you are still in SSMS you can simply right click and hit start
Note if you stop a SQL Server instance from within SSMS AND then exit SSMS and try and go back in you will be denied as SSMS is unable to connect to a service that is not up and running.
In such a case to restart the instance you need to go to SQL server configuration manager and click on the instance and restart from there.
In Windows 8.1 you can get to config manager by using the search facility.
Windows 10 Alternative
Hit the search ring
type in services.msc
A new dialog should appea that will have Services(Local) with several columns – Name / Description / Status / StartupType
You want the Status to be Running
Use the mouse to highlight SQL Server (MSSQLServer) and then right click
Start
Your server should now be running