You can check index.ini in the mail folder to make sure it's still present and make sure it still has entries in it. But, Opera is probably complaining that it can't find the mail folder or omailbase.dat that's in it. Or, even accounts.ini.
If you can goto "Menu -> help -> about Opera Mail", it should tell you where the mail folder and operaprefs.ini is at. Make sure the mail folder is still there. Also, check operaprefs.ini while it's closed. Under the [Mail] section, make sure there's no "Mail Root Directory" line so that Opera Mail uses the default spot on the OS for the "mail" folder. But, if you do have a custom mail folder location, make sure "Mail Root Directory=correct_path_to_mail_folder is correct.
You can try renaming operaprefs.ini so that Opera creates a new one when it starts back up. If that fixes it, the problem is in that file.