That’s the one.
Dennis Franz played 2 different regular characters on Hill Street Blues: the corrupt Detective Sal Benedetto in the 1982–83 season and, later, the main cast character Lieutenant Norman Buntz from 1985 to 1987.
Danny John Jules, who played Dwayne, left and returned as a guest, but then also did return for a full season, then left again.
I could’ve sworn at least one regular from House MD would have done this, but quickly checking IMDB I’m not so sure. Jennifer Morrison as Dr Cameron did disappear but came back only as a one off at the end so doesn’t count.
Not magically, this is discussed within the series, they were brought back by request with Goren having to undergo mandatory psychiatric sessions which are shown in camera. So it was not like nothing happened, what had happened before was discussed in that final season.
Law & Order: CI has another example though in this case is more of Law & Order universe thing with Chris Noth who was in Law & Order leaving in 1995 before returning in 2005 to Law & Order: Criminal Intent. Same character with the same history, technically a different series, though there has always been a link between the various shows in the franchise. In between the two he did a number of other things including being a regular on Sex and the City
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And then there was Dan Florek, who played Lt. Kragen for the first three seasons of Law & Order, left, and came back 6 years later to act in 333 episodes of Law & Order: SVU as Captain Kragen.
Looking up Cooter for a different thread:
During the second season, Ben Jones left the series for a few episodes due to a dispute over whether the character should be clean shaven or have a full beard. In his absence, Cooter’s place was filled by several of Cooter’s supposed cousins who were never mentioned before or since. Jones returned when the dispute was solved — Cooter would be clean shaven, although, for continuity the episodes were broadcast in a different order than that which they were filmed, so he was not clean shaven until the third season onwards.
Of course then I remembered the Coy and Vance era I had blocked out:
Cooter didn’t want to ruin his image in Congress
If we can count a span of several episodes (but not necessarily an entire season) as an absence, I suspect there are a few that occurred because one of the stars went on maternity leave.
The only one I can think of off the top of my head is Gina Bellman, who left Leverage for a term. Jeri Ryan stepped in as a different character who pretty much filled the same function. Bellman returned for the remainder of Leverage and continued in the role in Leverage Redemption.
Jane Leeves (Daphne) missed four episodes during season 8 of Frasier when her pregnancy got too big to hide. They sent her off to a “spa” where Niles remarked she had “lost 9 pounds, 12 ounces.”
Alyson Hannigan (Lily) missed only two episodes of How I Met Your Mother. In the opening scene of the last episode before she went on maternity leave, the character was supposed to have been so offended by a joke that she walked out, leading the narrator to comment,“And we didn’t see Lily again for 4 weeks.”
George Takei left Star Trek for a while to be in The Green Berets.
I don’t know whether Aloma Wright from Scrubs would count. She played Laverne until 2/3 of the way through season 6 (Episode 15, “My Long Goodbye”). Then she returned in the second episode of season 7 as a different character, Shirley (“My Hard Labor”).
IIRC, the producers thought that season 6 would be their last season, so they killed off a major character, only to regret it once the show was renewed.
ETA: Ah, no it doesn’t pass the “left for several seasons” criteria in the OP. Though she does return as the original Laverne in the final episode (“My Finale”).
Another one from The Jeffersons…
Paul Benedict, Bentley, wasn’t on the show for a little over one season and then came back.
Ancient history, compared to the above, but Bea Benaderet was Cousin Pearl Bodine on The Beverly Hillbillies in 1962-3, but the character disappeared when Benaderet got to star in Petticoat Junction. Both series were created by Paul Henning. Eventually Benaderet’s Petticoat Junction character of Kate Bradley appeared with Beverley Hillbilly characters and with those on Henning’s Green Acres, all part of the Henning-verse, in a process he called “cross-pollination”. Benaderet had appeared on the radio show Granby’s Green Acres, which Henning had also created.
Barbara Bel Geddes missed multiple episodes in season 7 of Dallas, then had multiple bypass surgery. Producers signed Donna Reed to replace her as Miss Ellie in what was supposed to be a multi-year contract. When Bel Geddes had recovered enough to return to work, the producers fired Reed from her contract and brought back Bel Geddes. Reed sued for breach of contract, but died before the case was resolved.
Maggie Roswell left The Simpsons for three seasons over a contract dispute. By the time she returned, they’d already killed off one of her characters, Maude Flanders.
Would you count Elizabeth Sladen as Sarah-Jane Smith in Doctor Who?
Series regular 1973-76.
Guest appearances in 1983 and 2006
Then became the central character in a spin-off series, 2007-11
Gap of 31 years.
But she didn’t return as a regular to the same show.
If we added WWE RAW to the mix there there are a few dozen pro wrestlers who left and then came back…The Rock, CM Punk, Brock Lesnar, Chris Jericho, Shaun Michaels among others
It’s borderline, but it is a connected show, part of the same fictional universe.
Up to the OP if he accepts it.