The satellites will not be falling out of the sky, they are 11000 miles above the earth. But a major solar flare, a large dither put back in by the military, or jamming by those are enemies or those who think of it is a practical joke. Certain electronic equipement have jammed ship board GPS's.
Plus you may have not plugged in the correct chart datum. Or position the GPS antenna in the optimum position on your vessel. Bad antenna connection have caused one cruise ship to run aground. And I've had three GPS's go belly up on me, which is a good reason to have all of your way points listed in your Navigation work book AND use secondary means of Navigation to confirm your position.