Based on the details you’ve provided, there isn’t a clear or definitive answer. The two most likely explanations are:
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Data was deleted by you, another user on the device, or possibly by an app or extension.
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Your Windows encryption (DPAPI) was corrupted or lost. This encryption system protects sensitive data such as saved passwords. If the encryption key becomes inaccessible or damaged, the passwords technically still exist in your Brave profile folder, but the browser can no longer read or display them. You can find a bit more on that at https://support.brave.app/hc/en-us/articles/29808985123085-Sensitive-data-storage.
Beyond those, there are too many unknowns to draw a firm conclusion. You only mentioned that your passwords were gone, but there’s no information about whether there were recent updates, reinstalls, program or extension changes, Windows version differences, or the exact Brave version in use. With so little context, everything else would just be speculation.
Even with all of that information, it still would be tough to provide any answers.