Neutral wire is not working
You have a open neutral somewhere. If you know what circuit that issue is in on:
 Look too see if there's anything plugged into any receptacles and unplug it ( or them ). If you find there was, then go back &amp; check to see if there's still "hot" power on that white wire...
If there's no longer power on that white wire, shut that circuit off then open up the closest receptacle box ( pull out the receptacle ) and inspect it.... 
If the above didn't find it, go to the next receptacle and the next and so on until you find the culprit.

Open up every box related to that circuit. Even your main panel box....and you'll check to see if all the "whites" are connected on the common terminal... That's a start, and hopefully a "finish" too.

Good luckBe safe.