A process serving application pool 'Classic .NET AppPool' suffered a fatal communication error with the Windows Process Activation Service

I am running an ASP.NET 3.5 web application on IIS 7 on Windows Server 2008.

I received the following error earlier this afternoon:

HTTP Error 503. The service is unavailable.

I looked in the Event logs and discovered that:

A process serving application pool 'Classic .NET AppPool' suffered a fatal communication error with the Windows Process Activation Service. The process id was '3328'. The data field contains the error number.

Here is the full details of that event:

Log Name:      System
Source:        Microsoft-Windows-WAS
Date:          18/09/2009 14:58:31
Event ID:      5011
Task Category: None
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      computername
Description:
A process serving application pool 'Classic .NET AppPool' suffered a fatal communication error with the Windows Process Activation Service. The process id was '3328'. The data field contains the error number.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="Microsoft-Windows-WAS" Guid="{5 .. 0}" EventSourceName="WAS" />
    <EventID Qualifiers="32768">5011</EventID>
    <Version>0</Version>
    <Level>3</Level>
    <Task>0</Task>
    <Opcode>0</Opcode>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2009-09-18T13:58:31.000Z" />
    <EventRecordID>23552</EventRecordID>
    <Correlation />
    <Execution ProcessID="0" ThreadID="0" />
    <Channel>System</Channel>
    <Computer>computername</Computer>
    <Security />
  </System>
  <EventData>
    <Data Name="AppPoolID">Classic .NET AppPool</Data>
    <Data Name="ProcessID">3328</Data>
    <Binary>6D000780</Binary>
  </EventData>
</Event>