Publishing a website for a client. Home page was working fine, now has an error.

Any page we try to have as the root page (URL showing in Umbraco as "/") throws the error below.

Any sub page we publish shows up fine.

We didn't do anything to the page, I promise. We did upgrade our hosting to a higher site memory allocation, but nothing else there changes either.

Please help!

Umbraco v 4.7.0 (Assembly version: 1.0.4090.38017)

asp.net 4.0 

Windows server 2003 / MYSQL / IIS version 6

Stacktrace:

[NullReferenceException: Object reference not set to an instance of an object.]
   umbraco.requestHandler..ctor(XmlDocument umbracoContent, String url) +4431
   umbraco.UmbracoDefault.Page_PreInit(Object sender, EventArgs e) +1062
   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.EventHandler.Invoke(Object sender, EventArgs e) +0
   System.Web.UI.Page.OnPreInit(EventArgs e) +8876158
   System.Web.UI.Page.PerformPreInit() +31
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +328