My App pool went down. what will be case?,

Hi All,

My app pool went down and after which main site went down. What would be root cause?

Below is the event log, SharePoint log found:

08/27/2014 16:37:01.79 w3wp.exe (0x18BC)                      0x0894 SharePoint Foundation         Configuration                8059WarningAlternate access mappings have not been configured.  Users or services are accessing the site https://mainsiteurl with the URL https://SecondWFEserver.  This may cause incorrect links to be stored or returned to users.  If this is expected, add the URL https://SecondWFEserver as an AAM response URL.  For more information, see: http://go.microsoft.com/fwlink/?LinkId=114854"/>

Thanks.

Badri