Twitter requires that you re-authenticate your account in third party social schedulers every time you change your password and/or your permissions in your account. Lately will give you a reminder via an email notification and within your dashboard in your Channels & Messaging library. Any Twitter account that needs to be reconnected will say "Authentication expired - please reconnect" next to the account name. To reconnect your Twitter account in Lately, go to Setup --> Channels & Messaging.
Select “Connect Channel" and click "Twitter”. We’ll automatically update the token for any channels you’ve already connected, and ask if you want to connect any others.