Have you attempted to re-register? Is the version in use the same one that you purchased? Click HELP | ABOUT to confirm.
It is possible that the software auto-updated, and thus your license is not valid for that version.
Reinstall the original, or a version that was released within six-months of purchase, then register it if necessary. Find older releases here. Ignore virus warnings.
If these measures fail, get in touch with NCH.