Hello
I am in your position more or less...🙁
It is difficult to say what the issue may be without some electrical measurements starting with measuring the battery's voltage while it is connected to the speaker to ascertain that a ~7.4V (nom) is delivered. If you have a multi-meter (DVM) you can take this measurements. You can charge the battery fully and then you can measure the battery's voltage prior to connecting it to the speaker (A fully charged battery will deliver ~8.4V). If the voltage drops significantly once you connect the battery to the speaker then one of the protection components in the speaker's inlet may have shorted out.
If the battery voltage is delivered correctly then there may be an issue with the speaker's inner PSU circuitry (It utilizes several power components to derive different voltages) or the BT module itself (My speaker is suffering from this issue - the chip simply stopped reacting and that was that).
You also mentioned that you left the speaker "plugged in"...I gather you mean the AC grid. So there may also be a short in the AC/DC unit inside the speaker (it is a separate PCB). Though I would not expect the device to be affected when operated on battery alone in such a case as there are usually two different supply rails even if they feed the same circuit.
In any case, from my (bitter) experience, the problem usually resides in the PCB/s. In such a case you should be ready to open the speaker up or seek out someone with the technical know how that can help you do so.
