Please try the following:
1. Reboot your computer.
2. Try NodeXL again.
Does it work now? If not...
3. Uninstall NodeXL. You can do this by going to Control Panel, Programs and Features; searching for "NodeXL" in the Programs and Features search box; right-clicking each NodeXL item that was found; and selecting "uninstall" from the right-click menu.
4. Download and install NodeXL again.
Does it work now?
These are really just stabs in the dark, because I don't know the actual cause of the message you're getting. Perhaps something got corrupted when you had to shut down Excel.