Hey!
The mystery crate is no longer in the store, did you claim this from
/cow? If so, it should appear as an enderchest in your inventory.
If you purchased this mystery crate key through the store or cubitshop, please make a support ticket here
https://manacube.com/support/ under the “webstore support” section, and provide the transaction ID from the email confirmation (if taken place via the webstore).
The current crate is the Valentines Day crate, any keys purchased for the monthly crates should be this theme.
Hope this helps!