Need help finding account.

I bought a new Xbox series X because my old one was severely playing up. I signed into my account and found every thing has gone, every game I downloaded has vanished. If I go to download them again from the Xbox store, I have to pay for them again. Obviously I must have another account but I have no clue what the email address and password is. How can I find it, any help most appreciated because at the moment my Xbox series X is empty,I had downloaded about three hundred games and I can't pay for that lot again. Sorry for waffling on.

:)