Is it internal or external antenna Sam?  If external I'd start there checking the cable for continuity and the fittings for good connections. 

Some GPSs have a memory clear function,  basically it's a hard reboot that resets the memory to the way it came from the factory.  With one of ours it needs that about once a year or it constantly throws fake anchor alarms. 

What make/model is it?  Someone else with a similar unit might have a better idea.

I had the same problem and had to replace the atenna. If you can see the top of the antenna (mine is on top of the radar mast so not so easy to do) you should see a green light as I recall. If its yellow or red, thats the problem.

I don't know what brand you have, but here's my recent story:

My brand new $3000 Raymarine (C120w) would occasionally flake out.  The Raymarine people told me that it was because the internal GPS in the chartplotter was not getting a clear sky view.  I asked if the external antenna would work any better and they would skirt the issue.  I think they were simply avoiding the truth of the matter, that an external antenna worked better and the internal antenna was not a good design.  Since the sky view of my chart plotter was only blocked by a thin layer of Sunbrella, I tried adding an external antenna.  I've never had the external antenna fail and the reception performance numbers are consistently higher.