This occurs when your EndNote can longer find the license key file or registry entry. Sometimes a reboot is all that is needed to resolve the problem, but if it persists, you'll need to reinstall the software.
Backup any EndNote libraries you may have and any customized output files, import filters, or connection files. (Generally libraries stored outside of your EndNote program folder should not be affected by the reinstallation.)
Download a fresh copy of the installation file from VT's Network Software portal. (This will also give you the latest main version.) We recommend doing a full install with all output files, import filters, and connection files. After installation, use the Help menu, Check for updates to see if any minor versions have been released since the main file was loaded by Network Software.