I had the same problem.
while I was on my bike I accidentally pressed some button combination
possibly JBL Connect + Volume up
It didnt turn on again.
then I opened it up, rooted around without any result and closed it up again.
not very difficult
BEFORE you open it up try the reset option
plug it in, as soon as you hear the "here I am" sound
remove the cable and press
Volume UP and Play / Pause buttons at the same time and hold them until a hard reset occurs.
here are some button combinations for the charge 3 I found on reddit
JBL Connect + Volume down
says the firmware number and blinks battery level diodes matching said numbers (coincidence?)
Volume down + Bluetooth 10sec
turns off inbuilt EQ (and limiter?)
JBL Connect + Volume up
weird mode, possibly FW update mode
(to go back, connect to powersource and hold the combination again)
Volume up + Play/Pause
resets the speaker
lets all bug JBL about releasing the button combinations for Flip 4 as they are nowhere to be found