Electrical problem!

[SideShow Bob]

Is this light on a 3 way setup (2 switches on 1 light) ? If so the other switch is probably in the opposite position from what it has been, and the 3 way circuit is miss-wired.
Especially if this is a fairly new dwelling to you or if the second switch is in a place where you seldom use it.
I had a relative that lived in a house for several years and all of a sudden had a similar problem, turns out one of their kids had thrown the seldom used second switch.
Found that the original build electrician or a previous owner didn't know how to wire up a 3 way switch circuit.

A quick way to tell if it is on a 3 way is to look at the switch, if it has Off/On embossed into the switch lever it is a single switch circuit. If the switch lever is blank ( no embossing ) it is a 3 way switch.