Why Facebook application id suddenly become invalid?
If you're sure the app is not in sandbox mode (which was gema.megantara's answer), and hasn't been deleted for policy reasons by Facebook the only explanation I can think of is that you've restricted the app demographically (i.e by country or age)
If that's the case the API won't return the app's details unless you make the API call using a user access token for a user who meets whatever restrictions you've applied, and users who don't meet the requirements won't be able to use the app.
If that's what you've done, you can remove the restrictions via the API (via the restrictions field of the Application object or in the ''Advanced'' tab in the app settings in the App Dashboard
If using the frontend interface to change the settings, it's the ''App Restrictions'' field you want to edit, screenshot attached 
Basically the same solution as the accepted answer, just updated for the latest FB developer site has been updated so you need do this:
- Apps->Your App
- Status+Review
- Do you want to make this app and all its live features available to the general public? -> Yes
