my fit was working perfectly until 6.64.
a) how can I install the old firmware that worked?
b) how can I reset my One so it will work?

my symptoms are identical to 
https://community.fitbit.com/t5/One/Fitbit-One-Will-Not-Turn-On/td-p/2566778

this is CLEARLY a problem with your firmware
1) do you have a engineering fix? AKA ask the engineers how this was broken and how to fix it?
2) IF NOT, please provide me with contact to reach your lawyers ... you intentionally broke my working device, this is illegal

IF you cant fix this, I have to assume that your products will always be disabled by you in the future.
Time to find any other supplier.

thanks for posting LINKS HERE on how to trouble shoot this problem
so the 1000s of other with the same problem wont have to call the help desk