Sharepoint - Application Pool is auto stopped when browse web application in IIS7
Check whether your credentials for your domain has changed and you did not apply it.
If this is the issue, change the following: - Open app pool advance setting > Identity > custom account > Set (give user name and passowrd)
I found that the problem I had that resulted in a 503 was due to the App Pool being set to support 32-bit applications.
I went to IIS manager, app pool, advanced settings, and set "Enable 32-bit applications" to "False".
This was on a Content App pool, though, not Central Admin.
1) Open IIS.
2) Right click on your Application-Pool for the web application and select "Advanced Settings".
3) Set "Rapid-Fail Protection" -> "Enabled" to False.