See the procedure described here to reset the device Bluetooth pairing: https://community.fitbit.com/t5/Charge-3/Charge-3-is-not-pairing-with-Bluetooth/m-p/3354691/highlight/true#M50473
It is most likely a PSU failure and/or some capacitor blown. You need a multimeter and a new PSU (like this: https://www.ebay.co.uk/p/5007142744), or if you have a configurable power source you could use it to verify that the problem is the PSU.
Find an authorized center here: https://www.fluke.com/en-us/support/service-center-locations Or if you are in Australia: https://spares.bigwarehouse.com.au/index... Another online shop: https://www.myflukestore.com/category/fl...
These are indeed 5-sided Torx Security, size 15; however the shaft of my multibit seems too fat to fit in the plastic canal.
Which multibit (precision?) screwdriver are you guys using? The canal seems to be just 8mm wide.