There are many reasons for this and if no code changes or changes to SQL-Server.
Try changing from Windows Authentication (if that is what is used) to SQL Authentication
In SSMS (SQL-Server Management Studio)
- Right click on the server name and select properties
- Select security tab
- Enable the SQL Server and Windows Authentication mode
- Restart the SQL Server
Otherwise reverse the above, SQL Authentication to Windows Authentication.
Another idea, if passwords are used, ensure they are not expired by logging in as admin to SSMS (SQL-Server Management Studio)
Also see the following which is not resolved but has things to try out.