Go to your Pictures Folder and find the iPhoto Library there. Right (or Control-) Click on the icon and select 'Show Package Contents'. A finder window will open with the Library exposed.
Look there for a Folder called 'Import' or 'Importing'.
Drag it to the Desktop. *Make no other changes*.
Start iPhoto. Does that help?
If it does then look inside that folder on your desktop. Does it contain anything you want? If not you can trash the folder.