Restart the computer. Ensure you can log in to facebook via the web browser.
If the app requires additional permissions in order to access it from facebook, you'll need to set those.
In other words:
If you've made no setting changes in facebook AND you previously had access to facebook via the app, this should not apply to you.
However, if you've never had access to facebook via the app, or you've potentially changed a facebook setting further protecting your account, or limiting access to it, then the likely explanation would be a facebook setting restricting access.
Sometimes you have to approve an app to access facebook, but usually the app does that by itself after you enter in the credentials. I thought there was a section to allow access from windows phone, or iPhone, etc. somewhere in there.
Other than that, I just don't know