Well, the first Fitbit Flex my wife got died after 4 days. So we exchanged it out and 3 months later, it's not working. Been having issues but this time I saw that it showed 2 lights, 1 blank light and 2 lights before it went into try to charge. There is nothing in the manual for this and finding anything online is even worse as nothing is out there on this.
After researching all I could, finally called the company to see what they would do. With the device syncing up to their servers, they can see that the battery in the Fitbit is not holding a charge anymore and went bad. They would never say exactly what those lights meant but I'm going to associate it with the battery going out in the Fitbit, which is therefore a dead unit and unusable.
We were past the warranty but they wanted a receipt for when it was purchased. I didn't have it but knew the dates for when we bought it. Since they can see that we were on our 2nd device and when we connected it to their servers, it shows the dates when it was done. Since I knew the exact dates for when we purchased it, and they could see it in their system, they waived needing the receipt. So we have a new Fitbit on the way.