Q: I cannot get iPhoto to open today. I have shut down twice - still no success. Any suggestions?

1. See if there is a crash log you can post here.
2. Try a safe boot.
3. Try a test user account.
4. Boot into your recovery partition and repair your hard drive.
5. Repair permissions with Disk Utility.
6. If you have a Time machine backup, restore it
   1. if you don't make backups, start making backups.

Mac OS X: What is Safe Boot, Safe Mode?

Isolating an issue by using another user account

When to Run First Aid in Mountain Lion's Disk Utility - For Dummies

About Disk Utility's Repair Disk Permissions feature

Try deleting the com.apple.iPhoto.plist.

You need to look in your user Library/Preferences for the .plist. Either hold down the option key while using the Finder “Go To Folder” command and select your user Library in your home folder or use the Finder “Go To Folder” command and enter ~/Library/Preferences/com.apple.iPhoto.plist.  Move the .plist to your desktop.

Open iPhoto while holding down the command and option/alt keys and rebuild the library.

iPhoto Library rebuild

Open the application and test. If it works okay, delete the plist(s) from the desktop.

If the application is the same, return the .plist(s) to where you got them from, overwriting the newer ones.

If you prefer to make your User library permanently visible, use the Terminal command found below.

Show User Library

You might want to bookmark the command. I had to use it again after I installed 10.8.5. I have also been informed that if you drag the user library to Finder it will remain visible.