You can't delete the app, did you maybe unpin it from your start screen? Just swipe to the right to get to the list of your apps look for the app, then long press and choose pin to start
I'm not sure how that's possible? Could you explain it a bit more? Like you unpinned it from Start screen? All your accounts should be under settings and you can pin them back.
At least one Mail app/account must be there. You can also check settings under email+accounts. If this is empty just add mail account again. Good luck.