Hey, it depends on whether its not charging or if its not taking a charge.
If it is showing that its charging but after unplugging it you see that its not taking charge its very likely to be a battery issue. To solve that you would need to replace the battery(Available for many of the fitbits) but this requires some technical abilities.
If its not charging at all there is a vast amount of possibilities of what could be wrong but you could attempt to clean out any debris from the charging connector on both the watch as well as the charger.