Backup assistant is only for Verizon Wireless customers.
If the app is not in the app list, nor can be opened with Cortana, you'll have to do a hard reset.
The missing app cannot be updated or uninstalled via xap managers, but if you're lucky and have an SD card that the app is installed on, then you can manually delete the app folder via file managers by manually typing the folder directory.
The missing app cannot also be updated via Store, as this will also result in an update failure, so in short:
"If you're app is installed on the SD card, then you have a solution. Otherwise, do a hard reset."