Try accessing your mail via your provider's website, rather than using a dedicated mail app. Ideally do so on a different device (such as a PC or laptop).
If you can get in using your old password, then use the 'Change password' option to reset it and try again on your iPad with your new password. If you still can't get it, check the account settings to ensure that your Ipad app is actually trying to access a valid account. If all else fails, delete the account and then create it again.
If you can't get into your email via your provider's website then either:
(a) you're using the wrong password ; or
(b) there's a problem at the provider's end.
So you'd then need to contact your email provider in order to try to sort things out, as changing things on your iPad obviously won't help.