Don't give up just yet. I have the same unit and it has done it a couple of times as well. Not sure why, if it was radar as 'bee15' suggests or something else, don't know. Anyway, just unplug your unit from everything in the boat and leave it unplugged for 5 minutes. The unit will re-set itself and you should be fine. Sounds wacky, I know but worth a try.
My unit has continued to work fine after the couple of episodes that occured over a year ago.
This 'fix' has worked fine on my neighbours unit as well. He also had the same problem a few times but nothing since.