Here's verbatim what Fitbit Support sent me at the time. If this doesn't work, spend your time finding your purchase receipt and claim a replacement if its still in warranty, or try requesting a discount on a replacement if it's out of warranty.
"We're sorry to hear about the problems you're having with your Fitbit Ionic and we appreciate all your troubleshooting efforts.
In most cases, you can resolve the issue by giving your tracker what's called a "factory reset" and then setting up the tracker as a replacement on your Fitbit account. Note that a factory reset erases any data stored on your tracker.
To do a factory reset, follow the instructions:
1. On the watch, hold all three buttons for about 10 to 12 seconds.
2. When the Fitbit logo disappears, quickly release the bottom right button, continuing to hold down the left and top right buttons (releasing the bottom right button as soon as the logo disappears is time sensitive).
3. When the device vibrates, release all buttons.
4. Wait for approximately 1-2 minutes while the device clears all data. Eventually, the Fitbit logo appears and the device boots and displays Fitbit.com/setup.
5. When the watch successfully resets, Use the normal out of the box process of updating the firmware (See How do I update my Fitbit device?).
If you still can't sync your tracker please let us know, but hopefully this gets you back on track."
