Yeah, his son Karec had become sick with a pretty serious viral infection while his father was on-tour and died while at hospital and Plant's first wife sent him a letter breaking the awful news. The band itself was actually staying at a very posh French Quarter hotel which I still think exists and by that point, unlike previous 70's U.S. tours, lets just say Zeppelin's 1977 U.S. tour was a mixed bag, and that might be too nice of a characterization. While they still put on some great shows in Detroit and Seattle's Kingdome, their "Day on the Green" 1977 concert in Oakland saw Peter Grant and John Bonham beat the hell out of one of Bill Graham's roadies, who had supposedly slapped Peter Grant's young son earlier after picking up a sign backstage. Graham called the cops, arrested Grant and Bonham for assault and battery. This incident probably also was also the sign of increased tensions between Graham and Grant over Grant's rough, rowdy managerial style and the two had never been the best of friends. Eventually, charges were dropped but the 1977 "Day at the Green" show in Oakland turned out to be Zep's last-ever show in U.S. with original band (a pre-metal Judas Priest was actually Zep's backup band at that show and reportedly, Lynyrd Skynyrd gave one of their greatest shows ever this same day).
If you go back and watch fan videos on YT of many Zeppelin shows during the 76-77 time period, you'll probably notice that Plant spends a lot of time sitting down on a stool or chair and isn't that same demonstrative, powerful "rock god" of 1973-74 and that's due he and his wife getting involved in a near-fatal car wreck in Greece about 18 months before. The injuries Plant suffered were so severe that more then a few doctors told Plant while he may walk again, his mobility would be significantly limited. By the late 70's, its a bit of a minor miracle Led Zeppelin didn't break up before Bonham's death. Plant and Page weren't ever the best of friends but by 1978-79, their personal and working relationship had turned sour again and John Paul Jones had to do yeoman's job keeping the ship afloat. Plant had felt that the band had achieved all it was ever going to and with musical trends changing with advent of punk and new wave of British metal scenes calling out Zep's musical/creative integrity, Zeppelin didn't seem to fit too well in a rawer, meaner music U.K./U.S. music scene of the late 70's/early 80's.
And as talented and gifted of a lyrical genius Robert Plant is, hes not Roger Waters where he had the lyrical chops, cynicism, and creativity to "react" to.the punk scene with Animals and The Wall. Plant is a great lyricist, but could he or Page radically change the band's perception, mood, and songs to reflect a darker, grimmer sociopolitical reality like "Dogs", " Pigs", "Sheep", " Another Brick in the Wall pts. 1-2", "Run like Hell", "and Comfortably Numb" like Floyd did? No.