Seeing a prompt saying: Enter the password for the Exchange account?
Is the Mail app prompting you to Edit Settings or Cancel?
Has the Mail app not received new emails since the day(s) before?
These are symptoms of needing to refresh your authentication to Exchange.
Choose Cancel and then go to Settings app > Mail > Accounts > Exchange
At the top you should see the blue text saying Re-enter password... tap that and then choose Sign-in when prompted. The rest is self-explanatory from there.
Once completed, exit the Settings app and re-open the Mail app. You may have to swipe down to refresh. Worst case, force quit the Mail app and re-open it again.