Please update to TweetCaster 4.6.2 in the Play store.
Once you update the app, your access should be restored.
It turns out that Twitter detected what they thought was suspicious activity on our previous API token (the part of the code that allows the app to access Twitter), so they suspended the app.
We have generated a new API token, and have tested and it is now working again.
If you are still getting issues after the update, go to your phone's main screen, tap menu --> settings --> applications -- > Manage Applications --> TweetCaster and "clear data" and then go to https://twitter.com/settings/applications and revoke access for the TweetCaster application.
Then try reinstalling the app and logging in again - that should force a full revalidation of the app with Twitter. Unfortunately you will need to restore your accounts/settings again, but hopefully this will help.