Hi Indy R1..... I had a similar problem with a Garmin unit on a clients boat last summer.....
Spoke with Garmin (really helpful) who (IIRC the order of things) suggested we left it on for a while as it had been unused for a v.long time, when that didn't work we downloaded a software update on SD as they suggested, that didn't work, checked the NMEA T section /connectors (some were a bit dodge from sitting in a wet locker loose on the floor
) replaced those and no joy....
......which left the antenna (external model - not sure what yours is)..... We were sent out a new one, plugged it in and hey presto..... There was no visible damage etc..... seemed odd that it could fail like that (no moving parts???), but thats what it was!
Not sure if thats any help.....would def speak to Garmin..... Good Luck!