Open up the start menu, in the search type SQL Server, and this should bring up the DB related tools

Select SQL Server Configuration Manager

Once the manager is open, look under the "SQL Server Services" heading

You should see instances of SQL server, as well as a few other processes.

Ensure both the SQL Server instances as well as the SQL Server Browser service all report as 'Running'

If either of these are not showing as running, right click the item and choose 'start'