Cannot connect to sql server a network related or instance-specific

Note:-

1. Check SQL Server (MSSQLSERVER) Service running or not in services.

Step to Open Services

1. Press Wndows + R to open Run dialog box
2. Type services.msc in Run dialog box => Click OK
3. Services dialog box open

4. Click on Start the service

Now your SQL Server (MSSQLSERVER) Service is running and Connect to Server.