one of my accounts in outlook is now requiring a password to open it

If the account is in New Outlook for Windows, the password it wants depends on the mailbox provider:

• Outlook.com / Hotmail / Live / MSN: use the Microsoft account password for that email address.
• Gmail, Yahoo, iCloud, or another provider: use that provider’s password.

Try these steps:

1. Test the account in a browser first
   • Sign in to that same mailbox on its web page.
   • If browser sign-in fails, fix the password or verification issue there first.
2. Remove and add the account again in New Outlook
   • Go to Settings > Accounts > Your accounts.
   • Select Manage next to the affected account.
   • Remove it, then add it again.
3. If the prompt keeps looping, repair or reset New Outlook
   • Go to Windows Settings > Apps > Installed apps > Outlook (new) > Advanced options.
   • Try Repair first.
   • If that does not help, use Reset, then open New Outlook and add the account again.
4. If the account uses two-step verification
   • Some accounts may require an app password or provider-specific sign-in setup.
   • For Outlook.com accounts in Outlook 2016 or Outlook 2013, two-step verification can require creating an app password.

If the mailbox is a Microsoft account and the password is no longer accepted, recover the account here: Recover your account

• Outlook access denied, asking for password - Microsoft Q&A
• My username and password have stopped working
• Outlook 2016 or Outlook 2013 repeatedly prompts for credentials when adding Outlook.com accounts