Need a little detail please. our older UE Megaboom says "charge critical..." but when plugged in, the charge light does not come on. Plugged in, the device still does not operate. I tried some things here at IFIXIT and got the light to indicate charge.
1) Unplug the unit (this procedure doesn't seem to work if the unit is plugged into the brick)
2) hold the Plus button and the power button for a few seconds.
3) Plug it in to see if the light flashes (mine is blue-white)
alternate... you might try with the Minus button
alternate... you might try with both plus and minus and the power button.
I don't know which of the three worked.
However, if your little charge light flashes red or flashes but it doesn't charge there are other answers on this site that might work for you.