I have a 3 year old 5d mark II with magiclantern installed for around 1,5 year. It has been working without issues til few week ago.
I had the LiveView on and I noticed the battery was blinking so I decided to change it for a new one. Camera didn't switch one that day after the change.
Few days later it switched on perfectly but when I changed the memory card, it stopped working turn again.
Unfortunately I didn't re-start battery each time I was changing the card since I was aware this was a best practice.
Camera doesn't turn on without card.