Flashing the stock image may have erased your backup. However, since the G2 supports an External SD card, I don't think it would have.
From the SD Card's root, it should be at ./TWRP/BACKUPS/. Restoring a backup from here using TWRP will re-root your device (If your device was rooted when you made the backup). You can also use apps like Titanium Backup to selectively restore parts of it... but Titanium Backup requires root.
You may, however, be able to selectively pull files from the backups - a full backup of your phone's filesystems.