You can try deleting autosave.win in the "sessions" folder in the preferences folder.
You can try renaming operaprefs.ini in the preferences folder.
You can check accounts.ini in the "mail" folder. If the section for the account you deleted is still there, you can try removing it.
If that doesn't work, you can edit index.ini in the mail folder and remove the section the specified the category and views for that specific account. Then, update the index count at the top of the file.
If none of that helps, you'll probably have to rename your mail folder, start Opera Mail, set up your accounts again and import your old messages from the renamed mail folder into each account via the "Menu -> settings -> import and export -> import mail -> import generic mbox" method. But, hopefully you won't have to do that.