The singer thought he was just lazy when he put on weight and stayed at home in his LA mansion, and put it down to burn-out from his previous party lifestyle.
But a trip to the doctors revealed he was actually suffering from a hormone imbalance and he is urging others who might be feeling lethargic to get themselves checked out.
He told The Sun: 'If you are lethargic and your energy is a bit zapped and you find stuff difficult to do, I advise you to go and check your hormones. I did it by mistake and a few interesting things came up. It explained to me why everything seemed to be so much of a hassle for such a long time.'
But Williams said that after getting medication and marrying Ayda Field, he got his mojo back.
He said: 'Thanks to the grace of God, or whoever she is, I stumbled across the answer and I'm, like, 'Now I'm ready to go!' I'm absolutely rocking. Life is wonderful at the moment, personally and professionally.'