Complex sentence: Not having sung for over five years he was petrified with fear, but was such a success that he was held over for six weeks, having regained his confidence, he accepted an engagement at the London Casino where he was a tremendous hit.
Split into simpler sentences: Not having sung for over five years he was petrified with fear, but was such a success that he was held over for six weeks. Having regained his confidence, he accepted an engagement at the London Casino where he was a tremendous hit.