Recently, my android signing key disappeared from Adobe PhoneGap Build. So I created a new one and signed the app with that. But now Google Play won't let me use the new build as an update, because it's signed with a different key. How can I get my old signing key back? I didn't delete it, the error is on Adobe PhoneGap Build's side.
You should always maintain a copy of your signing keys in a safe location in case you ever need them again (PGB does /NOT/ count; it's not a backup solution).
PGB doesn't let you download your signing keys, and for good reason. I'm afraid you'll have to release a new app or hope that you've got a copy of the signing key laying around on your computer somewhere.