Have you removed the batteries?
I would remove the batteries, wait a few seconds, then press the power button (as if turning it on, to discharge any power still in the JBL) and hold it for a few seconds.
Then put the batteries back in and hold the power button the usual few seconds to turn it back on.
I've never experienced this problem myself, but that's the first thing I'd try if it was me. Good luck!
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