When this happens the most common cause is the battery just being dead, this can happen for a variety of reasons such as excess heat (being left in the sun) or just general use.
It could also be the charging port however if you get led indication that the charger is working then you can probably rule that out.
You can replace the battery but that may be a very involved process due to how tight and sealed these products usually are,
However sometimes people have luck leaving devices like these on charge for a while for it to randomly come back to life but I wouldn't get your hopes up doing this.