The template tree will not expand for my umbraco install. This install has been up and running fine for a long time, but something must have been corrupted. How can I restore my template tree?

I just tried to create a new document type to automatically create a new template, and the following error occurred:

[NullReferenceException: Object reference not set to an instance of an object.]
   umbraco.cms.businesslogic.template.Template.get_MasterPageFile() +104
   umbraco.cms.businesslogic.template.Template.getMasterPageContent() +14
   umbraco.cms.businesslogic.template.Template.setupTemplate() +178
   umbraco.cms.businesslogic.template.Template.GetAllAsList() +225
   umbraco.settings.EditContentTypeNew.bindTemplates() +209
   umbraco.settings.EditContentTypeNew.Page_Load(Object sender, EventArgs e) +118
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   umbraco.BasePages.BasePage.OnLoad(EventArgs e) +15
   System.Web.UI.Control.LoadRecursive() +50
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627

A .master file was created for this new document type in the masterpages folder.

Help very much appreciated.

Tom