most wallets hide your addresses when they are used and now have 0 balance to discourage address reuse [1] but these addresses (the keys) will always belong to you and be a part of your wallet. depending on what kind of wallet you use, there is always a way to get those keys out. and if the balances change from 0 they will reappear in your wallet.

[1] https://en.bitcoin.it/wiki/Address_reuse