.aspx webpages not working in MSW2k3/IIS6

Hello,

Using:
   * Microsoft Windows Server 2003
   * IIS6
   * ASP webpages & ASPX webpages

I'm having trouble getting .aspx webpages to work properly.
Yesterday I was getting  this error...

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Server Application Unavailable
The web application you are attempting to access on this web server is currently unavailable.  Please hit the "Refresh" button in your web browser to retry your request.

Administrator Note: An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur.
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I was looking thru IIS & application pool settings......didn't make any changes and then........it started working.
Now today it is not. I thought the issue was in IIS and an application pool setting...not sure.

If it is an appplication pool setting.....what do I change?

Thanks,

---- Mike