Value cannot be null

When trying to open the Sitecore client (ie. by putting /sitecore in the URL), I got the following exception:

Exception: System.ArgumentNullException
Message: Value cannot be null.
Parameter name: item
Source: Sitecore.Kernel
at Sitecore.Diagnostics.Assert.ArgumentNotNull(Object argument, String argumentName)
at Sitecore.Web.UI.HtmlControls.Menu.AddFromDataSource(Item item, String target)
at Sitecore.Web.UI.HtmlControls.DataContextMenu.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I noticed that the URL had changed to /sitecore/shell

With so few clues as to the cause, I simply entered the full path to the log-in screen (/sitecore/login). The log-in screen loaded and I was able to log in successfully.

Out of curiosity, I logged out again and then re-tried /sitecore on its own. The log-in screen opened OK that time. I've no idea as to the cause and I don't fancy investigating. If you're having the same problem, why not try what I did? Good luck.