If you plug your fitbit into your charging cable and then into your computer, lights should start pulsing. (See here for details.) If they don’t start pulsing, then your fitbit will not take the charge. How can you fix this?
Take a look at the three very small circles at the bottom of your fitbit. They should be a shiny copper colour if they are clean, and they need to be clean in order to take the charge. Ideally they should always stay clean, but it is easy for them to get dirty since they are so close to your skin.
To clean them, you need a very gentle abrasive to clean them. What worked for me was taking the end of a paper clip and very gently scraping the circle until I could see the shiny copper again. Then I plugged it back into the charging cable that was attached to my computer and the lights started pulsing.
Note! If you are the least bit concerned about breaking your fitbit by doing this, then do not do this. Instead, take it back to where you purchased it and see if they can help you. You are responsible for your own fitbit. That said, this worked for me and my fitbit worked fine afterwards.