Your mnemonic seed is the 13 word private key that is generated when you create a new wallet. When you first created your wallet, you would have received the below message:

"Take Note of your Private Login Key!
Below this you will find your thirteen word "Private Login Key". Keeping this secure and private is very important, as it is the only way that you will be able to login to your MyMonero account. As we don't store your private login key on the server there is no way to recover it if it is lost! Your private login key can also never be changed, and if it is stolen or otherwise compromised, you will have to move your funds to a new account with a new private login key. Therefore, it is best that you backup your private login key by writing it down, perhaps obscuring it as part of a poem or letter, and storing it in multiple safe and secure places."

Unfortunately, if you have not followed those instructions there is no way for us to recover it. We don't store the key, nor did we ever know what it was, it was only on your computer.

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