bands or musicians who used to be popular, then were mediocre or forgotten, then hit it big again

Yeah, I was a lonely Sparks fan when that album came out, and it did get played, and was even featured on MTV. Still, it was far from a genre-defining opus, or really one of the best albums ever made, as Lil’ Beethoven in 2002 was. And it was after the latter album that they started regular touring even in far-flung places, that persists to this day.

The Allman Brothers were huge by the time the Fillmore Album was released. Duane dies and then they release Eat a Peach. Still riding high but they slowly slipped down the infighting and drug trials to absolutely ruin what they had. As albums were released the band was so poorly received by the 80’s (along with accusations of internal betrayals) they hated each other so much that they broke up and vowed never to record with one another. Some years pass and then in 90’ the band gambles and regroups and the band releases “Seven Turns” and get back on the road. They not only won their audience back but picked up another generation of fans and critics. They become bigger than ever and ride out their career on their terms until just a few years ago. They had it all, lost every bit of it and then fought their way back. Quite a story.

In the US, Queen.

While they remained more or less consistently popular in the UK and much of the rest of the world up until and through Freddy Mercury’s death, they completely disappeared in the United States for most of the 1980s. They had two #1 hits in 1980 with “Crazy Little Thing Called Love” and “Another One Bites the Dust,” but their next albums/projects Flash Gordon and Hot Space were not well received, and they last played in the US in 1982. They basically did not exist to Americans until the Wayne’s World movie came out in 1992.

I think Rick Astley has to come into the list, as a serious competitor for the ‘most unlikely comeback’ award.

That’s a bit of an overstatement. You’re right that Hot Space was not well-received, but The Works, though not a mega-hit here, was something of a return to form, and I distinctly remember seeing the “Radio Ga-Ga” video and hearing “Hammer To Fall” played on the local rock station. And of course many Americans saw their performance at Live Aid.

One thing I love about the 80s, when I musically came of age, is that there were so many classic acts of the 60s and 70s who were releasing vital new music that got played alongside the work of newer acts, either because they were experiencing comebacks of a sort or because they just kept on keeping on.

Forgotten then a spectacular comeback, never mediocre:
Compay Segundo

Many of the rock bands of the late sixties and early seventies seem to have gone through a period of making it big for several years, then going throuhh a period of decline, often with band members dying or have substance abuse problems (or both, but not in that order) or leaving, and then coming back for another spell of relative fame, and in some cases repeating the cycle once more. The pop press was always saying that so and so was washed up and finished because he or she had not produced a new LP for a couple of years or more. Or that a particular performer had changed his style completely and had a new audience. Plenty of those, Bowie and Madonna come to mind, for starters.

The Beastie Boys were everywhere in the 80s, then disappeared in the 90s and 00s, but then started showing up again in the 10s. Mostly, I think, due to nostalgia from my peers.

What about Ozzy himself? Big with Sabbath in the 70s, as a solo artist in the 80s, took the 90s off, came back as a reality TV star in the 00s

Queen was very popular in the U. S. The local rock station was always playing their latest hit, right up to Headlong just before Freddie’s death. And of course his death was big news, comparable to Bowie’s or Prince’s deaths.

The Righteous Brothers came roaring back from the oldies station to top 40 airwaves when their 1964 song Unchained Melody was used in the 1990 movie Ghost.

Check Your Head (1992) and Ill Communication (1994) were pretty big with Hello Nasty (1998) not quite so big. The song Sabotage was particularly popular.

My theory is that he finally ran out of ladies’ undergarments and needed to start touring again to restock.

King Crimson had an earth-shattering debut with Court of the Crimson King and wound up with three great mid-'70s albums: Larks’ Tongues in Aspic, Starless and Bible Black, and Red. Then they went on hiatus. Six years later a whole new KC came back strong with Discipline.

On the classical front, how about Vivaldi? Superstar in life, reputation dwindled in death, revived in the early 20th century, now a star again.

When Hello Nasty came out there was a huge buildup, the single Intergalactic was everywhere (top 40 pop hit, won 2 Grammys) and the album went to #1 selling 681,000 in the first week alone.

One of the Righteous Brothers, of course, had a #1 single a few years before that.