Android API key for GCM is suddenly invalid? Unauthorized (401) error

I created an Android API key for GCM Push Notification for my Android application.From the last two days, GCM server returns Unauthorized (401) response in PHP.

Is there any reason for Android API key invalid/expire? Or Is Android API key deprecated by GCM?

Still I'm not getting proper explanation regarding Why Android API key is invalid? Most of them prefer Server key instead of Android API key for GCM but not mentioning as to why not use Android API key.

Why do not use Android API Key in GCM? Is there any limitation for Android API key? Or Is GCM server blocks the notification/GCM server is making Android API key invalid if notification message exceeds per day limit?