One gmail account has suddenly disappeared from my iPad but still works on my iPhone.

Have you configured the Gmail account on your iPad to authenticate by way of OAuth Token - or a password?

Google have changed their authentication requirements for mail accounts configured for password authentication - and for non-Google mail clients, now requires use of an App Specific password for each mail client/device. Accounts configured on existing devices may continue to accept your google account password until you make any changes to your account - after which the requirement to use an App-specific password will be triggered.

Once triggered, you will need to generate a unique password for your iPad Gmail account - that will differ from your normal Google Account Password. AOL, Yahoo, Google, Microsoft now all use App Specific Passwords when using anything other than their own email client Apps.

If you need to generate an App Specific Password, you’ll need to log-in to your Mail Providers account management webpage - using a web browser - and generate the Password from the appropriate account security page.

More information about AOL’s App Specific Passwords can be found here:

https://help.aol.com/articles/allow-apps-that-use-less-secure-sign-in?guccounter=1

For Yahoo, more information can be found here:

https://help.yahoo.com/kb/SLN27791.html

For Gmail:

https://support.google.com/mail/answer/185833?hl=en-GB

Microsoft:

https://support.microsoft.com/en-us/account-billing/using-app-passwords-with-apps-that-don-t-support-two-step-verification-5896ed9b-4263-e681-128a-a6f2979a7944

BobSquiers Said:

"One gmail account has suddenly disappeared from my iPad but still works on my iPhone."

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Use a New App Password:

Do you use an App Password? If so, try a new App Password. Sometimes, changes (i.e. updates) are made to the iPad, and then mail server syncing becomes an issue (which is what a new App Password it intended to fix). Go here for gmail: Sign in with App Passwords - Google Account Help - Google