If you just have the one set of wires in switch box then you do not have a hot and common.

What you have is a switch leg.

See

http://www.selfhelpandmore.com/switchoutlet/basiclightswitch/basiiclightswitchsl.htm

It is in the text, but not shown on the picture. The white wire is suppose to be remarked with tape or paint to show that it is a hot and not a neutral.

Also note that sometimes that this is wired wrong, with the white and black wires in the switch leg reversed. That does not affect how it works. Just confuses the next person that works on it.

If the switch was on then what you measured was correct.

If this is the case and you have verified that the bulb(s) are good then the problem is that either the neutral feed to the ceiling box is bad or the fixture failed.

Assuming you are really checking between hot and neutral, what you describe is indicative of a lost neutral. Check all your connections at the switch box, the light, and at other switches or receptacles on that circuit.

Thanks for the replies. When I realized that both Neutral and Hot were at the same potential it dawned on me that means it's either a short, which should have blown the breaker, or that Neutral and Hot somewhere back along the line must be attached to the same Hot source. What I found was the Neutral had disconnected from a plug and fallen across a Hot wire. Therefore, they were the same potential.