"We'd like to confirm that we have identified a problem and are in the process of finding a solution," Microsoft wrote in a statement. "We will make the game available as soon as we can, and we apologize for the inconvenience! Thank you for your patience in the meantime."
5th Cell creative director Jeremiah Slaczka wrote on the game's forum: "We are working with Microsoft to fix these server issues. Many people from both 5TH Cell and MS are working together to sort the problem out. We are making progress, but it will take time unfortunately. Sorry for the inconvenience, however we don't foresee this problem to continue once it's fixed."
We've been in touch with Microsoft for an estimated time of a fix. We'll update as soon as we hear more.
Update 2: Hybrid is back on the Marketplace.
Update: 5th Cell has told Joystiq the issue has been fixed, and the game will be back on Xbox Live tonight. Creative director Jeremiah Slaczka issued this statement:
"We have worked with Microsoft to diagnose and resolve the server issue surrounding Hybrid. Our entire team appreciates the overwhelming response of fans logging in on day-1 and for bearing with us during this short downtime. We expect a smooth gameplay experience from here on out."