Use "Force Quit" from the -menu to force quit iPhoto, and then launch iPhoto again. Has it stopped importing? If not,
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.