I suppose a fallen god would be a god that has lost his or her divinity or immortality. Kind of how a fallen angel is one who is no longer an angel, a fallen god has lost whatever it was that made him or her a god.

fallen = an adjective 1. having dropped or come down from a higher place, from an upright position, or from a higher level 2. degraded or immoral 3. overthrown, destroyed, or conquered: a fallen city gods = a plural noun 1. supreme beings, the creators and rulers of the universe 2. images of deities; idols 3. superhuman beings or spirits worshipped as having power over nature and human fortunes An example of 'fallen gods' could refer to Hollywood stars like Michael Jackson who was idolized by millions until the scandals of him being involved with young boys , made his popularity drop to a very low level.