Today, in my SharePoint Site, Create, Edit Menus under Site Actions, New Menu, New Users menu - basically all New menus stopped working in my libraries and lists!!!
The action that we had performed earlier was that - we had stopped a just started backup (using the stsadm command). The stsadm command had just locked the site and before the backup could complete, we had closed the command window.
So all we had to do was to unlock the site collection using
stsadm -o setsitelock -url http://servername -lock none
This was a great lesson!!!