During the log-in process, some players may accidentally create a secondary account using a different e-mail address than what is connected to their primary account. Remember: no two MTG Arena accounts can share the exact same e-mail address. Identifying a secondary account can be done before and after logging into Magic: the Gathering Arena.
Before Logging In
Upon entering the start-up screen, if the MTG Arena client was not previously set to automatically log into a player’s primary account by clicking on the “Remember Me” option or the player is signing onto a different device, they will see the Register or Log In option.
After Logging In
Upon completing the Color Challenge/unlocking all Play modes with a new secondary account, players will note that items acquired on their primary account are not available for access and usage.
If a player has created a secondary account on a supported device that is different than where they usually access their primary account the Display name will be different. Even if the names are the exact same they will have two different 5-digit identifiers at the end (such as SameDisplayName#12345 and SameDisplayName#54321). It is not possible for a single MTG Arena account to have a Display Name with two different 5-digit identifiers.
If you still believe that your MTG Arena account has been reset, please contact Wizards of the Coast Customer Support, which can be done through this website here.