The CSS SQL Server Engineers have a guide for solving connectivity issues:
In addition to providing a quick checklist of items that you can go through, the doc provides step by step troubleshooting procedures for the following error messages:
A network-related or instance-specific error occurred while establishing a connection to SQL Server
No connection could be made because the target machine actively refused it
SQL Server does not exist or access denied
PivotTable Operation Failed: We cannot locate a server to load the workbook Data Model
Cannot generate SSPI context
Login failed for user
Timeout Expired
The timeout period elapsed prior to obtaining a connection from the pool
Click through for the guide. It’s in choose-your-own-adventure format, though without nice graphics.