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Try changing regions and logging in.  Since Diablo 3 is Global play each account has access to all of the regions.  Characters are region specific so the characters may have been created in a different region.

If you login to Diablo III and cannot find your old heroes, it is possible you have logged into the wrong region. Diablo III has "global play," which means you can change the region your games are created in. However, progress in each region is completely separate, which means progress in one region will not carry-over to another region.

To change regions in the Battle.net App:

1. Exit Diablo III completely.
2. Locate the region dropdown menu on the Diablo III page and change this to your desired location.
3. With the correct region selected, click Play.

If you do not see the dropdown list above the Play button, you can change the region through D3:

1. Launch Diablo III.
2. Press ESC to open the Game Menu.
3. Click Options, then the Account button on the left.
4. Pick the correct region from the drop-down menu on the right and click Accept.

It is possible that the account is under another email than the one you remember. Try logging in with another email address to see if you can access the account. 

If you feel the account was compromised, you may also try performing a rollback on the account. We urge you to use this service with caution because it can result in unintended losses and is limited to three total rollbacks per lifetime of the account.