I'm grasping at straws on this.
Launching PGP Desktop might possibly resolved this by starting PGP services if they are not already started.
If you haven't already, I'd try rebooting.
If neither of the above help, and this is a consistent problem, I'd try a re-install.
If these suggestions do not help, it may be helpful to state your operating system, what PGP version you are using, and specifics as to when this error occurs.