If your BURST Water Flosser is not charging the first step with the BURST Water Flosser is to try a reset. This is done by plugging the unit in to charge, Once charging go ahead and hold down the power button, After a couple of seconds all of the lights will illuminate on the unit. You can then release and the lights will go off after a second and the unit will then start charging.
Let it finish the charge and use the unit as normal.
If this reset has not resolved the issue then submit your warranty claim through the BURST website here: https://www.burstoralcare.com/register