There can sometimes be a mobile sync issue that occurs on iPhones around the time of a password change. The fix is to remove your account from Outlook Mobile so it can be added again.
1. Locate and open the Outlook app on your iOS device.
2. Tap the profile picture in the top left corner, then tap the account you want to make changes to
3. Tap the Settings gear icon located at the bottom left corner of the app.
4. Under Settings, choose General, then Remove Account
5. Verify you want to Remove the account from the device
Please note that removing an account from Outlook will delete all associated emails and data from the app, but it won't affect the email account itself or any data stored on the email server. No mail will be lost in this process. If you need to access the account again, you can simply re-add it to Outlook. You can find a Knowledge Base article on how to do that here: How to Set Up Outlook App on iPhone