The sad news came courtesy of the singer's PR firm, which announced her death on Tuesday -- but which didn't disclose any official cause, or the exact circumstances. Her children also confirmed her passing to various outlets.

Melanie is, perhaps, most famous for singing at the OG Woodstock back in '69 ... where she crooned as one of just a few solo female performers that year, and left quite an impression on the audience. In fact, her experience at Woodstock went on to inspire more music.

One song she ended up writing in the wake of that was 'Lay Down (Candles in the Rain)' -- and it apparently had to do with people doing exactly that at the historic gathering.

A couple years after her big Woodstock gig ... Melanie came out with a single that put her on the map, and it was called 'Brand New Key,' which went on to become her biggest hit.

The song ended up being featured across a number of different mediums over the years -- most notably in 'Boogie Nights' during Dirk Diggler's porn audition with Rollergirl. If you've seen the movie, you probably remember this well ... it left an impression.

'BNK' was also heard in 'Jackass 3-D' ... as well as on shows like 'It's Always Sunny in Philadelphia,' 'The Bridge' and 'Doctor Who,' -- not to mention on late-night shows, plus 'American Idol' and 'The Voice.' Indeed, it was very popular through the decades.

Other tracks she'll be remembered for ... 'What Have They Done to My Song Ma,' 'Ruby Tuesday,' 'The Saddest Thing,' 'Bitter Bad,' 'Ring the Living Bell,' 'The Nickel Song,' and more.

Melanie continued working in and around music late into her life -- including meeting up with stars like Miley Cyrus and other contemporary folks/rock singers who carried on the torch.

Word is ... Melanie was actually working on a new album as recently as this month, one in which she'd be doing covers. She was 76.