Use Force quit from the -menu in the upper left corner of your screen.
> Force Quit
Then restart your Mac.
If the problem persists,
Select the iPhoto Library in the Finder. By default it is in your Pictures folder.
- Ctrl-click or right-click the iPhoto lIbrary. Select "show package contents" from the panel, that will pop up.
- Look for a folder named "Importing" or "Import". Remove this folder, but don't touch anything else.
Then launch iPhoto again.
If iPhoto now is working again, check the folder you removed, if there are any photos that you want to keep and save them.