Chuck was an early version of Dawn from Buffy the Vampire SlayerThe monks, before lucking out in the early 2000s by finding a Slayer with a family, frequently incarnated the Key, moving it around to keep Glory from tracing its energies. In this instance, it located them within a humdrum family situation with an associated member who may have had some kind of latent mystic powers. But as Fonzie's rep grew, attention began to be paid to that small section of Milwaukee. So it was that one day, when Chuck left for school, the spell was reversed. Only Fonzie retained any memories of Chuck, describing him to Al in a flashback, although Al must have been confused when Howard denied ever having known a Chuck. Finally, even Fonzie lost these memories as he rapidly filled Chuck's place in the Cunninghams' lives.
Chuck did more than just ascend the stairs...He Ascended to a Higher Plane of Existence.
Fonzie killed Chuck.In order to become a more powerful Ensemble Darkhorse, Fonzie had to get rid of Chuck and take his place. Breakout Character? He forcibly made himself one.
Chuck's still in his room.Due to not having a lot of screen time, brother Chuck may have suffered from some kind of mental illness the audience didn't get to know about. This being The Fifties, the stigma may have been great enough that Chuck voluntarily withdrew from society, or his family kept him upstairs to save face.
Chuck didn't exist at all.The first two seasons were All Just a Dream.
Fonzie is a Ta'veren male channelerHe used Balefire on Chuck to pull him out of the Pattern. He also uses channeling to get the jukebox to play.
Fonzie is a Technopath.The Fonz has shown repeatedly that he can pretty much make any device work by either tapping it, punching it or even just clicking his fingers. If you check out the Technopath trope, you will see that Fonzie fits this trope quite well.
There is some sort of Worm Hole in suburban Milwaukee.Hence the '70s-style hairdos and attitudes seen in the late '50s-early '60s; the gang were continually slipping back and forth about 20 years. About 20 years later, it manifested again in the nearby town of Point Place, causing one Eric Forman and his friends in the late '70s to occasionally act and talk more like the cast of Friends.
Chuck Cunningham was taken by Mork.He then altered everyone's memories so they never missed him. Chuck mentioned his brother Richie a few times. This explains why Mork decided to visit Richie of all people on Earth and take him back to Ork.
Chuck was absorbed by a Doctor Who style space-time rift in the Cunningham's Attic.It explains his dissapearance and the cast forgetting him perfectly!
"Roger Phillips" was SUPPOSED to be a returning Chuck Cunningham, but Garry Marshall called an audible at the last minute.The more I think about this one, the more sense it makes.
Chuck died in a car accident and it was never mentioned on-screen.Probably the only logical explanation, given Howard's "two children" remark in the series' finale. How? There was an incredibly tragic accident where Chuck and his super-cute girlfriend were driving when they were rear-ended by a drunk driver. The collision is so violent that Chuck's car is pushed into the driveway of a gas station, where a gas tanker is refilling the underground tanks. The whole place explodes into a huge fireball, and there are mass casualties. Chuck and his girlfriend are literally incinerated, as are the gas truck driver and several customers and employees inside the gas station.
"Richie Almost Dies" was actually intended as "Richie Dies."The first half of the episode goes according to the original plot – Richie seriously injured in the motorcycle crash, everyone gathers at the Cunningham home, Leather sings "Find Strength in Your Friends" to Richie clips, and Fonzie makes his tearful plea with God to let Richie live. After the latter scene comes the suspensfully taken commercial break ... and then back to the hospital room. Instead of what actually does happen (it's morning, Howard and Marion walk in to see Fonzie sleeping on the chair, Richie wakes up and all is OK), Fonzie is still sleeping ... when Richie goes into a massive seizure. Fonzie wakes up, panics, and immediately – within less than five seconds – a half dozen doctors and nurses are in the room to aid Richie. Howard and Marion also show up and are crying, begging the doctor to save their son. But Fonzie is shouting and screaming, pleading with the doctors to save Richie, but it quickly becomes apparent that Richie is about to die. And he does die (offscreen, as the scene fades as the doctors are feverishly working in vain to save Richie, with cries from Howard, Marion and Fonzie). The next scene is the funeral scene – and after 15 minutes of just watching mourners file past Richie's casket (and him lying in state) – we fade to the funeral. There, Fonzie begins to give a eulogy, but less than three minutes in, he begins to break down, has a complete emotional collapse and has to be helped to his seat (by a teary-eyed Lori Beth and Ralph). So now, the Cunninghams have buried not one, but two sons.
The Fonz is from MarsThat is why he could break out of Mork's control when he froze everyone. It is also how he can keep up with him and his powers. Martians have similar powers to their sister race from Ork, they just activate them differently (such as Fonzie's snapping and quick hits). This is also why Fonzie is always after women. Mars needs them, remember?
Chuck was gay!and disowned by the Cunninghams, that's why he's never mentioned again and Mr.C comments that he only has two children. Makes sense in the context of a show about the 50s.