i OS backups made in i Cloud and i Tunes back up your mail settings, but not your emails. If Mail asks you to enter a password for your email account, make sure that your password is correct.
If you delete or change your email account settings, previously downloaded email might be removed from your i OS device. To check your email address and password, sign in to your email provider's website.
To make sure that your account settings are correct, compare the settings in the Mail app to the settings for your email account: If you have a POP3 email account, then you can only check email on one device at a time.
Close other email programs and sign out of webmail sites that you have open.
But just in case it’s not, here are a few things to consider.