I use my 6D2 at least 2-3 times per week with a Canon Battery grip attached... Suddenly it would not turn on.
I thought that it might be that the batteries had been exhausted so I recharged both batteries.
The camera still would not turn on! I then tried each battery in my 7D2 and both were fully charged and each worked just fine/
I then removed the battery grip from the 6D2 and inserted the batteries individually. Neither worked!