Hey Oli!
Nice to meet you! I'm Darhyl, an Independent Advisor. I'm sorry you are experiencing this on your Minecraft account. No worries, let me help you sort this out.
In order to find the Microsoft account where your Minecraft account was migrated, you will need to sign into all of your Microsoft email accounts on account.microsoft.com > then go to Payment and Billing > Order History.
From there, check if you have any Minecraft transactions. If you have found the transactions, use the same Microsoft account to log in to Minecraft Launcher.
Thank you for your kindness and understanding! Please let me know if you need anything else.
Best regards,
Darhyl