I would recommend turning the unit off and removing the coax antenna cable. Check your connection for corrosion. If it looks good, take a multi-meter and check the OHMs value of the GPS antenna (GPA017) from the center pin to the outer part of the connector. It should read roughly 270 ohms. If the reading isn't close, then you most likely have a bad GPS antenna or problem with the GPS cable. If the antenna tests well, then your unit would need to be checked out by a qualified dealer. In Greece, your distributor is:
FURUNO HELLAS S.A.
Fleming 95 & Amissou Keratsini, 18757, Piraeus, Greece
Phone: +30 210-4004426