User is unable to connect to SQL Server in 2016. What may be scenarios and how to troubleshoot it?

Possible Scenarios and Resolution to login issue with Fixes-

• •  Error: 26: SQL Browser Firewall No connectivity between client and server.- Check SQL Server browser service using Services.msc 
  
  
  •  Error: 28: Instance TCP/IP was disabled- Go to SQL Server configuaration manager and enable the TCP/IP 
  
  
  •  Error: 40: Instance service is not running - Open the services and RUN the MSSQLSERVICE to permanent FIX run type make it as automatically. 
  
  
  •  Error: 18456: Login failed. (invalid login or password ) - Check your user name and pasowrd, Also server which you are trying to connect.
  
  
  •  Expired Timeout: Network issue, Server is busy, In server max sessions are open ,No available session memory - Try to ping the server, if network issue contact to Network team and if Memory issue contact to DBA support team or increase the memory. 
  
  
  •  Connection Forcibly Closed: Update the client computer to the server version of the SQL Server Native Client.
  
  
  •  In single user mode: if any other service is connected with the db Engine, it doesn't allow connections. change the mode as multiuser