You have just demonstrated why you always do Live Dead Live before working
on anything that can hurt you.
Live. You first put your voltage checker on a live circuit and confirm tha
t it does indeed detect that the power is on. Otherwise, you can get a zer
o reading from the circuit you are about to touch, but you have no idea if
your detector has died. Which they obviously do, frequently.
Dead. You deenergize the circuit you are about to work on, and then confir
m with your meter that it is actually off and not being backfed from somewh
ere else, actually on a different breaker, wired backwards, etc.
Live. And then you test your meter AGAIN on a known live circuit. Because
as you've proven, meters die all the time, and it may have been dead when
you checked your supposedly deenergized circuit.