I didn't know where to find it in the database.
Your CSS changes are stored in the *_posts table, along your Posts, Pages, and other Custom Post Types.
Custom CSS revisions use the safecss post_type and post_title, so you should be able to find them by filtering all posts with this Post title.
Re-enabling jetpack didn't bring it back.
You will need to connect Jetpack to your WordPress.com account for the Custom CSS editor to appear again. Once you have done so, you can access your previous CSS revisions thanks to the Revisions links in the bottom right corner:
http://i.wpne.ws/QY42