We're rather confused as to what's going on but it seems as though Sonic CD has joined Super Monkey in becoming unavailable on the Windows Phone store. Both titles were published in the Nokia Collection of apps for Lumia owners to enjoy for free, which was seen as an awesome move by Nokia. It's therefore come as a slight surprise that both games can no longer be downloaded.
The error provided to users is the pesky "this app is not available for your device." We could assume that there has been a mistake, there were bugs found, or the deals were only limited for a handful of days. We'll wait to see if they should return, but Nokia failed to respond to our requests for clarification on the issue.
If you were lucky enough to get in while you could, good job. For the rest of us, we'll have to fork out on the Xbox Live versions - which isn't necessarily a bad thing. There are other games available in the Nokia Collection so we hope that these popular titles return soon enough.