Another possibility....sometimes there is a glitch in the clock timing. As most GPS units actually never do a full reboot, but stay in warm standby, they assume their clock is nearly in sync with the satellites, and then fail miserably to get a signal if they are out of sync.
This has happened regualrly to me on marine GPS units from Garmin and others (lowrance was one, i think).
Powering off and on never seemed to make a difference.
I spoke to some technical suppotr guys at garmin, who gave me the forced resync process for the units. This takes a good 5+minutes to resync properly, and they tell me that after that, you really need to make sure the unit is on for a minimum of an hour to make sure it caches the correct time and movement for the satellites in view (which it then can extrapolate the others from).
This all sounded somewhat dubious to me, but i have to admit, ever since then, if I have lost a signal, a hard reboot has got things back working within a few minutes.
The shame is that for all the units I use (handheld, pocket, marine) the reboot method is different, and seems to be so on every type, even within the garmin range.
Ah well, still love GPS, and thankfully have never lost both GPS at the same time :-)