The short answer is if you do not have the original private key and certificate that your app was signed with then you must start over with a new pk/cert pair. In addition to the mobione signing doc you can learn more from this android doc on the topic http://developer.android.com/tools/publishing/app-signing.html
I realize this is a pain. We took for granted that users would read the docs on this process and never use the developer mode pk/cert provided by mobione for their production use. To prevent this scenario we have recently modified MobiOne app center builder to no longer create non-debug mode apps when a user does not provide an explicit pk/cert.
Last word on this is just as our docs and the android docs call you must take extra special care to never loose or expose your signing private key and certificate. We see users being way to casual on this matter. It’s serious!