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by using the "final getsuga tensho" against Aizen, it uses ichigo as the getsuga, thus why the technique is called "final". He loses his power directly afterwards in the manga, but is able to keep them for a little while longer as they slowly fade in the anime.

He is able to regain a new kind of spiritual power known as fullbring later on, however, and has been working to regain his shinigami powers through it. it's better if you read the manga to keep up.