Please use Settings / Already Purchased? Google Play:
You can reinstall Poweramp Full Version Unlocker any time for free as long as you use the Google account you bought it with. Note that for Google Play Store purchases, if you have multiple google accounts on the phone, your "original" Google account should be selected in Play Store (side menu / top-right white arrow next to your account).
The Unlocker should also be available for you to install from your list of previously-used apps.