Do you have the XA Ultra or the XA2 Ultra? If the former, this should probably be in the XA forum, not the XA2 Ultra forum, based on the generation of the phone.
Lave it charging on a wall outlet overnight. Then press and hold Power for about 20-30 seconds. If nothing happens, then it could be failure of the battery, Power button, or motherboard. Is the phone still under warranty?