Complex sentence: Barbara Bel Geddes' health had caused her to miss half of the seventh season, and after the season finale, she left the series entirely, with the role of Miss Ellie recast with Donna Reed for season eight.
Split into simpler sentences: Barbara Bel Geddes' health caused her to miss half of the seventh season. After the season finale, she left the series entirely, and the role of Miss Ellie was recast with Donna Reed during season eight.