Donna Summer -- the Queen of Disco -- died this morning (Thursday, May 17th 2012) after a battle with cancer.  She was 63 years old.  Sources close to Summer say ... the singer was trying to keep the extent of her illness under wraps.

Summer was a 5-time Grammy winner who shot to superstardom in the '70s with iconic hits like "Last Dance," "Hot Stuff" and "Bad Girls."

She continued her dominance in the '80s with "She Works Hard for the Money" and "This Time I Know It's for Real."

Summer and her producer Giorgio Moroder defined the dance music era.

She married Brooklyn Dreams singer Bruce Sudano back in 1980. They had two daughters together.