I couldn't summarize it anybetter than this wiki already did, so I will just quote it below:

"Saruman, being a Maia, did not truly die. His spirit separated from his body much like his master Sauron's after the Downfall of Númenor. As an incorporeal spirit, he should have been called to the Halls of Mandos, but the tale implies that he was barred from returning. Tolkien indicated that his spirit was left naked, powerless and wandering, never to return to Middle-earth:"

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