I've got a similar problem with my older garmin 298 it will work fine for ages and then just loose all the satellites. When I first turn it on it goes through the startup and starts to acquire the satellites but they won't turn green and lock in, I usually hit the menu button then select auto locate and within a couple of minutes it is working as per normal.
Mine has been doing this for 18 months or more and I do run a second gps as a back now since the time it stopped working offshore with no land marks in sight.