Message Ubisoft Support with the account name and email address that was attached to it.

Hello Sneed, thank you for getting in touch with us and sorry to hear you have lost your key and account details.

Please contact Ubisoft Support over one of the following links (privately) -

Twitter Support - https://twitter.com/UbisoftSupport
Facebook Support - https://www.facebook.com/UbisoftSupportUK/
Ubisoft Support Website - https://www.ubisoft.com/en-gb/help/contact

Please make sure to let us know as much of the following details as possible, when you do -

-Proof of purchase screenshot for the game, from Steam or other
-Login Name
-ManiaPlanet Email Address

We will then do our best to recover your account details for you allowing you to log in,

-Ubisoft Support