When you say "not working", do you mean that when you place it on the camera shoe and press the shutter, it does not fire?
Try using your camera's flash control menus to set Master Flash to Enabled.
The xenon tube may have failed. It can be replaced by a technician, but it may be too costly to return to YongNuo for servicing.
Is the Pilot light showing Ready? If not, try a brand-new set of Alkaline batteries to ensure that it is not a battery problem.
I suggested "brand-new alkalines" to test. It seems a pity to not try this simple test before dumping the flash, especially as using the old batteries in a new flash might produce the same effect.Used different batterys. All lights come on and sometime it struggle to come on.