There is lots of reasons why an update may hang, but I don't know the exact cause of this one. Try downloading and installing the latest version from the MSE website:
Update it directly from MSE. If you do not have the icon on your taskbar.
Go to search, type Security Essentials, then click on it, then go to the update tab.
EDIT: Its not a virus definition, so you cannot update from the AV itself, sorry.