i have a black & decker automatic charger, and the few times i,ve had to charge a completely dead battery my charger wouldnt even start charging it. the way i got it to charge was i hooked up a 12v light to the battery and then started the charger. with the light drawing a current from the charger it was enough to get the charger to work. then after the battery was partly charged i was able to remove the light and let it charge on the lowest amp. this has worked a couple of times for me after i left something on and discharged the battery untill it was dead. the last time i did it was in my truck. the glove compartment light was left on. the charger just kept giving me a code the battery was bad. i turned on the dome lights and started the charger. the dome lights came on real low at first, but the charger was working. i only had it set on 4 amps. after the charger charged for awhile i turned the dome lights out. the battery charged up and i am still using it.

as for what caused your battery to go dead i dont have a clue. it is possable the battery just went bad, it happens. but if you can get it to charge back up and it holds the charge then something was probably left on. if you have a float switch on your sump pump, make sure something doesnt have it stuck in the up position. good luck.
sherman