As long as a user knows their recovery seed, typically a 12- or 24-word list that was initialized with their wallet, they will always be able to restore their wallet.
A wallet will store many addresses and private keys as you send and receive bitcoin, but you only need to backup your wallet once. Because an HD wallet creates addresses deterministically using the same seed, all of your addresses can be backed up using the mnemonic word representation of that seed, also known as a recovery phrase.
Likewise, if your recovery phrase is exposed to someone, they have the ability to steal all of your funds. Keeping the recovery phrase safe from strangers and ensuring you do not lose or forget it are equally important.