If you have changed the password for your email, you’ll need to change the password in two places on your iPad/iPhone:
Settings > Passwords & Accounts > [mailbox account] > Account > Password
Settings > Passwords & Accounts > [mailbox account] > Account > SMTP > [Primary Server name] > Password
The first setting relates to receiving email from your IMAP/POP3 mailbox; the second relates to sending mail using SMTP. Mail failing to send, after a password change, is usually indicative of the SMTP password not having been updated on client device.
The other method of updating your email credentials, a somewhat more “blunt” instrument, is to delete the email account from your device - then re-add the account. Assuming that your using an IMAP mailbox, nothing should be lost - and all your email will reload from the server.
I hole you find this information to be helpful in resolving your email issue.