If you backed up your private key, you can recover your account using the Recover Account feature.
Memo cannot recover your account if you lose your password unless you have backed up your private key.
Memo uses secret private keys for accounts to interact on the blockchain. User's private keys are encrypted with their password so that even Memo cannot access them. If a user's password is lost then the private key cannot be decrypted making your account inaccessible.
For users who have forgotten their username, you can still use the account recovery feature. Only the private key is required. If you forgot your username/password combo and do not have a backup of your key, then unfortunately your account is lost.