Subscriber details are stored on the WordPress.com site and are normally accessible through the Jetpack → Site Stats page in your WordPress dashboard (near the bottom). That assumes that your site is still connected to WordPress.com, and that the "subscriptions" module has not been deactivated. For more information, please review: http://jetpack.me/2012/01/17/how-to-view-your-email-subscribers/
Alternatively, you can sign into your WordPress.com account and view the subscriber information there. Jetpack would have automatically prompted you to create a WordPress.com account when you set the plugin up. If you forgot the password, you can request a new password by entering your email address here:
You can also reset the connection to your site from within the WordPress.com site.