Where is my Bitcoin balance stored?

If you open your wallet and see a number you didn't expect, your balance didn't vanish: it was never stored as a balance.

Your balance is not a stored number

The Bitcoin blockchain has no accounts. There is no table anywhere that says 'this address has X bitcoin'. What exists are unspent outputs—UTXOs—that were recorded in previous transactions and that your keys can move.

The balance you see in your wallet is just the sum of those UTXOs. As the source puts it: UTXO model — Bitcoin whitepaper + Core docs.

How your wallet calculates what you can spend

When you open your wallet, it doesn't read a balance. It scans the blockchain for outputs your keys can unlock and adds them up. If it misses one, the number it shows is lower than you expected.

That's why if you restore your seed in another wallet and see zero, it doesn't mean your bitcoin disappeared: it means that wallet isn't seeing the right UTXOs.

What it means that the blockchain has no accounts

Having no accounts explains why you can't 'recover your balance' as if it were a stored value. What you recover is the ability to see and move those outputs.

If your wallet doesn't show what you expect, the problem isn't that your balance was erased: it's missing information, like a different derivation path or an address gap.

No accounts doesn't mean your wallet can't be wrong

Source: Modelo UTXO — whitepaper de Bitcoin + docs de Core