Sharepoint 2013 with http error 503

We have a machine dedicated for SharePoint 2013, and running on Server 2008R2SP1 and MSSQL 2012.Domain Controller is running on other machine.

At first the problem was that: if it accessed from outside, it showed "http error 500" (accessing internally was always fine), we tried to fix it by modifying the "A record", and then the error became "http error 401 - unauthorized". The workaround for that was to type the full path, i.e., and that worked for both internally and externally. However, 2 days after that, accessing it internally and externally, showed "http error 503 - Service Unavailable"

Any suggestion to fix it?
Who is Participating?
Rainer JeschorConnect With a Mentor Commented:
503 service unavailable normally indicates that the IIS app pool / IIS site is not running.
Therefore please open IIS manager on the Sharepoint server and start the app pool.
Second review the Windows event log to identify the issue why the pool stopped.
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