he lost his powers when he used the final gensuga tensou on Aizen.The final gensuga tensou is a technique that when used the caster loses all his spiritual power (that's why it's called final).
He loses his power after he uses the final getsugatenshou. I believe it is due to him using all of his spiritual pressure that is the reason why it is called FINAL getsuga tenshou. as to what causes...