Excel Web Access: Error in communicating with Excel Calculation Services - exception: The request failed with HTTP status 503: Service Unavailable.

The excel services has stoped working properly on any SharePoint site, it used to work perfectly, now it's not rendering anywhere on the pages (farm wide). The follwoing is the error message being thrown on any report page or Excel Web Access webpart:

"An error has occurred.
Please contact your system administrator if this problem persists. "

and the follwoing is the error shown in the Event Viewer logs, follows and it's the same error in the SharePoint logs as well:
---
Event Type: Error
Event Source: Office SharePoint Server
Event Category: Excel Services Web Front End
Event ID: 5239
Date:  7/24/2008
Time:  4:58:59 PM
User:  N/A
Computer: <ServerName>
Description:
There was an error in communicating with Excel Calculation Services http://<ServerName>:56737/ShareServices1/ExcelCalculationServer/ExcelService.asmx exception: The request failed with HTTP status 503: Service Unavailable.
[Session: (null)
User: <DomainName>\userName].
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
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07/24/2008 4:58:59.57       w3wp.exe (0x0538)                             0x15C4      Excel Services                      Excel Services Web Front End        3skf      Critical      There was an error in communicating with Excel Calculation Services http://<ServerName>:56737/SharedServices1/ExcelCalculationServer/ExcelService.asmx exception: The underlying connection was closed: An unexpected error occurred on a receive. [Session: (null) User: <DomainName>\userName].
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I've explored many of the links available online, i had no luck to resolve this problem, can anyone help me resolve this issue?

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