The SQL Server evaluation edition is free for 180 days but when the evaluation period has expired, the SQL Server shutdown and remains offline.
To resume the SQL Server functionality, you need a valid license and perform the upgrade procedure.
Check if the evaluation period has expired
If the SQL service doesn't start, check the logs in the Event Viewer to see if the evaluation period has expired. The error message reported is self-explanatory. When the evaluation period has expired, the SQL Server will no longer work.
Enable the SQL Server
To restore the server functionality, you need first to download the SQL Server installation .ISO file then run the installation wizard.
From the installation wizard, access the Maintenance area and click Edition Upgrade option link.
Select Enter the product key and insert the purchased license. Click Next.
Accept the EULA and click Next.
After few minutes, the Edition Upgrade procedure completes successfully. Click Close to exit the wizard.
From Start > Run enter the command services.msc and click OK. Right click the SQL Server service name and select Start.
When the service status is reported as Running, the SQL Server resumes its functionality.
Now you can use the SQL Server Management Studio to establish database connection and check if everything is fine on your SQL Server.