Trying to use the iPhoto Upgrader to update my iPhoto Library on my external hard drive

I was trying to use the iPhoto Upgrader to update my iPhoto Library on my external hard drive in order to use the new Photos App. It wouldn't recognize the Library on my external hard drive, so I upgraded the original iPhoto Library on my iMac. This reverted iPhoto to my original Library & now it doesn't show the photos on my external hard drive. I've totally screwed this up & am afraid to move forward & further mess things up. How do I undo this mess?