I dropped by the hangar last night after our EAA meeting to try and further diagnose why the G5 wouldn’t power up. I called Stein on Saturday and he suggested pulling the backup battery, but I decided to just try powering it up again without changing anything. Low and behold, the unit powered up just fine! The backup battery was dead, so apparently it was keeping the unit in an odd state. A quick call back to SteinAir this morning confirmed that this is a known issue and Garmin has a fix on the way (if not already available).