Hi,
Have you tried using a different appropriate USB cable to see if it works?
Have you tried using a different charger or connecting it to a different USB port in a PC or laptop to see if it works?
Using a strong light and a magnifying glass check that the "pins" in the USB connector (charge port) of the speaker are all there and that they are straight and parallel with each other. Also check that the port is clear of any lint or dust. If there is any obstruction in the port do not use any type of probe, especially metal, to try and get it out. You may damage the pins. Try short sharp bursts from a vacuum cleaner.
If all the above proves negative, here is a link to the Ifixit guide on how to replace the battery. It is handy as it shows how to open your speaker so that more testing can be done to ascertain what the problem is.
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