You never made a single backup of it? That's the very first thing you should do when creating a keystore. I have mine on dropbox, google drive, a USB drive, and emailed to myself.
It's kinda hard to give advice without knowing the type of app. Two options that I can think of:
1. Unpublish the current app. Republish it with a new keystore and be prepared to handle many unhappy user emails (in which you'll need to ask them to buy the new version which you will issue a refund immediately).
2. Re-brand the app as a version 2.0 with many enhancements, publish 2.0 and tell users it's a different app.