Two-Factor Authentication is a widely used secure way of using Email. Outlook is not two-factor friendly just yet. If you are using two factor authentication and wondering why your Outlook is no longer working, your email provider most likely had an update that you might not have been aware of. There are two options to get this resolved.
1. Disable Two-Factor Authentication
This option does reduce the security level of your email account and is not the best option, but will allow you to not have to worry about setting up an app-password which is the next option.
2. Setup an App-Password
This option allows you to login to Outlook with your email address and an app-specific password generated by your email provider to use. This password replaces your current password when asked for your credentials.
The two most common providers that have been pushing Two-Factor to their accounts are Gmail and iCloud. Click the below links to help generate App-Specific passwords for both these accounts.