anyone know what a 'Page Load Exception' is?

Last week, an admin user was logged in to the site (and left the site open - did not log out ), when he returned after lunch, most of the site content for him had disappered.
Yet for everyone else the content was still there. It was just for one admin user which the site content had disappered (gone blank).

I have got around this by deleting this account and creating another admin account, but I'm curious to know how it could have happened.

Further Info:
I think the admin user - may have visited napster whilst still being logged in to dnn site - could this cause a problem?

Having checked the log viewer - it displays the following error:

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AssemblyVersion: 04.05.05
PortalID: 0
PortalName: NEPOF
UserID: 24
UserName: nepof
ActiveTabID: 36
ActiveTabName: Home
RawURL: /dnn/Providers/HtmlEditorProviders/Fck/fcklinkgallery.aspx?FCKTheme=Default&tabid=36
AbsoluteURL: /dnn/Providers/HtmlEditorProviders/Fck/fcklinkgallery.aspx
AbsoluteURLReferrer: http://www.sypensions.org.uk/dnn/Providers/HtmlEditorProviders/Fck/fcklinkgallery.aspx?FCKTheme=Default&tabid=36
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: xxxxxxxx
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.UI.UserControls.UrlControl.cmdSave_Click
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.UI.UserControls.UrlControl.cmdSave_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.LinkButton.OnClick(EventArgs e) at System.Web.UI.WebControls.LinkButton.RaisePostBackEvent(String EVENT_ARGUMENT) at System.Web.UI.WebControls.LinkButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String EVENT_ARGUMENT) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String EVENT_ARGUMENT) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---
Source:
Server Name: xxxx
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AbsoluteURLReferrer:
http://www.sypensions.org.uk/dnn/Providers/HtmlEditorProviders/Fck/fcklinkgallery.aspx?FCKTheme=Default&tabid=36
Maybe there was a problem with uploading a link - but not sure if this can cause the site content to disapper?