The most important point first: Normally you shouldn’t press the G5 On/Off switches at all – neither for startup, nor for shutdown (when a 3 second shortcut is offered – the rechargeable G5 backup batteries last four hours each, so the extra 42 seconds really won’t make any difference).
The lower G5 (HSI) has occasionally been observed to not boot upon turning on the master switch. If that happens to you, the simple fix is to press and hold the affected G5’s On/Off switch for about ten seconds – just don’t let go – until it boots up.
That’s the only only situation where the On/Off button of the G5 units should be pressed. A recommended fix by Garmin has been applied on Jan 3 to hopefully keep that boot-up issue from occurring in the first place.