Hi, and thanks for responding. The version is 8.1.2. I tried launching (double-clicking) from the dock like I always do, and when that didn't work, I restarted and tried again from the dock (actually several times). Then I restarted and tried from the applications folder, then restarted and tried from the applications folder on the back-up hard drive. Then I went online, found this sight, saw someone else had the same problem and they were told to delete the com.apple.iPhoto.plist from the Preferences folder in the Library on their hard drive, so I did that before I started this discussion. I am not familiar with Spotlight.
I don't know if this is relevant, but when I had the app open last, I was deleting duplicate photos from the library.
I haven't tried rebuilding the iPhoto library as suggested above - I will do that this morning.
Thanks for your help!