This was working on Friday, but on Monday when I log into the backend of Umbraco this morning I see this error:

Server Error in '/' Application.

Root element is missing.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Xml.XmlException: Root element is missing.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[XmlException: Root element is missing.]
System.Xml.XmlTextReaderImpl.ThrowWithoutLineInfo(String res) +189
System.Xml.XmlTextReaderImpl.ParseDocumentContent() +1240
System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options) +58 System.Xml.Linq.XDocument.Load(String uri, LoadOptions options) +137 umbraco.BusinessLogic.ApplicationTree.LoadXml(Action`1 callback, Boolean saveAfterCallback) +151
umbraco.BusinessLogic.ApplicationTree.EnsureCache() +307
umbraco.BusinessLogic.ApplicationTree.get_AppTrees() +9
umbraco.cms.presentation.Trees.TreeDefinitionCollection.RegisterTrees() +129 umbraco.cms.presentation.Trees.TreeDefinitionCollection..cctor() +28

[TypeInitializationException: The type initializer for 'umbraco.cms.presentation.Trees.TreeDefinitionCollection' threw an exception.] umbraco.controls.Tree.TreeControl.Initialize() +228
umbraco.controls.Tree.TreeControl.OnPreRender(EventArgs e) +193
System.Web.UI.Control.PreRenderRecursiveInternal() +113
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Control.PreRenderRecursiveInternal() +222
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4297

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.36371