Under Settings, then to Yahoo mail, I tapped the "Reset All Accounts" button, now I can't access my Yahoo email. What do I do to get my email back to the way it was?

If it still shows up under your accounts you would want to tap on it and then tap on the email address. You should see the option to sign out, do so and then re-add the account by hitting add account from settings> Mail contacts and calendars.

If the accounts are not on the device start at re-adding the accounts.

If you need the server settings you will need to contact Yahoo.