You need to go to Control Panel / Administrative Tools / Services, and look for a service called MSSQLSERVER. Start the service by pressing the start button, and make sure that the startup type is set to 'Automatic'.
NOTE: Starting or stopping the SQL Server Service is not the same as starting and stopping the SQL Server. The Service ensures that SQL Server is running on the computer; however you can also stop and start the SQL Server, in order to do maintenance on the databases. This will prevent users from logging in to the server but will enable a database administrator, or consultant to work on the databases