The day before a two week vacation I went out to the garage to go to work and the LEAF failed to start up. It turned on but stalled half way through its start up sequence. After several attempts to start the vehicle I started to wonder about getting it to the repair shop. I couldn’t even get it in neutral to allow a tow truck to pull it out of the garage. It was a poser.
I went inside to consider my options. I soon realized that modern EV’s are like computers on wheels; a reboot often resolves computer issues, the same might work for the car. I returned to the garage, disconnected the 12 volt battery and left the car for 30 minutes.
Upon reconnecting the 12 volt battery the car started normally and drove normally. I chose to take it to the local Nissan dealer for it to be checked over. The dealer retained the car for two weeks while I was on vacation. Diagnostics revealed no fault. It started for them every day without fail and has for me since I returned. The failure to start is still an unsolved mystery. This is the first time in 111,000 miles the car has let me down in anyway, and thankfully I was able to resolve the issue myself easily enough.
I carry tools with me to disconnect the 12 volt battery in case it fails me again away from home.
Tesla Model 3 
I had a similar experience with my 2013 Leaf recently. I was unable to drive my cat to the shop, but once it was towed in, they quickly deduced that the 12 volt battery was nearly dead. Four years on the original 12 volt battery is not too shabby. The recent cold snap seems to have brought this about.
If you haven’t replaced yours in a while, it may be worth doing.
I did have the 12v battery replaced recently when the on board charger failed. So I don’t believe the battery itself was the issue, so far its a mystery as to what it was doing.
I had something like this happen to me also on my 2015 LEAF. The first time I was backing out of the driveway and it would not go into Drive and the screen went blank. I pushed the power button and turned everything off and when I powered back on everything came up like normal. The second time I was in the garage, had just unplugged and it would not power up when I pushed the button. So thinking like you did that the car reboots when charging; I plugged the LEAF back in, heard the thump, it did not need to charge so I unplugged it again and this time it started right up like normal. I almost have 30,000 miles and the only other problems I have had was a refrigerant leak in the AC line and a nail in one of the tires.
If you lose your 12v battery the car can’t turn on. The 12v battery supplies power to the contactor coils.