Complex sentence: He was made the abbot of Saint Michel-de-Tonnerre at some point after the year 1060, but he was unable to reform the abbey, which had become known for its laxity, and so he returned to Montier-la-Celle.
Split into simpler sentences: He was made the abbot of Saint Michel-de-Tonnerre at some point after the year 1060. Unable to reform the abbey, which had become known for its laxity, he returned to Montier-la-Celle.