Try the following to see if you possibly have a corrupted iPhoto library -
Launch iPhoto while holding down the option key on your keyboard.
Select Create New... from the dialog that appears.
See if it opens normally with a blank library. If it does, the problem is with the old iPhoto library.
If that is the case, first thing to do is make a copy of your old iPhoto library, so you will have a backup if anything further goes wrong. Then relaunch iPhoto again holding down the option key and selecting the old library. You will get the gear circle again. Force quit and then follow the steps in this Apple article for rebuilding the iPhoto library.
Best of luck.