Nostr: What happens if I lose my private key?

Short answer

If you lose the private key and every backup or signer that holds it, you cannot sign new events as that public key. A relay or client cannot reset the key for you. You can create a new identity and tell followers through any channels you still control, but older signatures and follows remain associated with the old public key. Plan a secure backup before this happens.

Sources

NIP-01: Basic protocol, NIP-19: Human-readable encodings.