Sorry to tell you the bad news, from the official Android developer docs:
Ensure that the app signing key is not lost. Loss of the app signing
key means that an app cannot be updated, so it is critical for it not
to be lost.
Update
It seems like Google now allow reseting certificates. Please fill-in an issue on this form. The operation may take a couple of business days.
Thanks @SushiHangover for pointing that out!
