Can't add password to Mail any more

One of my email accounts (using Mail) suddenly tells me it can't verify the account, then it tells me there is no password associated with the account. When I go to Mail > Accounts > Exchange [name of the account] > Account [my email addres], the next screen shows my email address and "Description" (Exchange) but there's nowhere to put in my password. It used to be there, but it's simply disappeared. My iPhone is running iOS 16.5 but the same problem has turned up on my iPod touch 6th gen running iOS 12. What's going on, and how do I fix it? (Right now I can access email on the iPod but not the iPhone.) Thanks for help in advance.