SQL DBA : Trouble shooting User Connection .

Scenario :

One of a user is unable to connect to SQL Server . What may be the possible scenarios ? How to troubleshoot it ?

Possible Scenarios

1. Error :26

                SQL Browser
   
                Firewall

                No connectivity between client and server

2. Error :28 

                Named Pipes & TCP /IP was disabled

3. Error :40

                If you are getting the error in Local then Instance service is not running in case of in Remote server then TCP / IP is disabled .

4. Error : 18456

                Login Failed .( Invalid login or password )

5. Expired Timeout

                 Network Issue

                 Service is busy

                 In server max sessions are open

                 No available session memory

6. Connection forcibly closed

                 Update the client compute to the server version of the SQL Server Native Client .

7. In single user mode if any other service is connected with the db engine then it doesn't allow                 connections.