Start by checking if SQL Server is operational on the database server.
If this is the case, check if the shared memory protocol / TCP-IP / Named Pipes is enabled on SQL Server. You can check this in the SQL Server Configuration Manager. If this is not the case, you must enable it, and then restart the SQL Server service.
In addition, if you use the SQL login to connect to the database, you must check if Mixed Mode authentication is enabled on SQL Server. To verify that you are logging into SQL Server and right-clicking the instance, go to Properties -> Security tab -> check if Mixed Mode authentication is enabled or disabled. . This will require restarting the SQL Server service again for the change to be reflected.
Check these 3 things first and let me know if this does not solve your problem.
Please flag this solution as a solution, if that solves your problem.