Hello Kkatada and welcome to the Conan Exiles forums. I think your enslaved NPCs are simply starving to death. Indeed, as with buildings and constructions, there is a decay time for enslaved NPCs. During your absence from the server you were not able to reset this decomposition time to zero and therefore your slaves died. I am deeply sorry for your misfortune and nevertheless wish you good luck.
PS:私は母国語ではなく、日本語から翻訳から抜け出しています。
PS: I’m getting out of translation, Japanese, not my mother tongue.