I cannot connect to my yahoo email server from my ipad

GIve it 24 hours. Yahoo has been 'upgrading' their servers, and if they're working on the one your mail is on, you lose access to your account via the mail app until they're done. You can try accessing it via mobile safari, that's what I did when it messed with my two accounts.

Ok. I know you said you deleted and readded it, have you tried it lately? I ended up having to do that to both of my accounts.

You could also try exiting your mail app. You do that by tapping the home key twice, your apps will come up in a band at the bottom, hold down on the mail app to make the red 'close' circle come up. Tap on that then tap on the home key again to exit.

You can then reopen your mail app and see if that helps.

Unfortunately, there's no singular magic bullet fix.