Without further information, my first guess is a corrupted SD card. Try the following steps.
1) With the SD Card inserted, locate and launch Disk Utility.app
2) On the left column lists of hard disks, DVD/CD and other storage devices detected on your MacBook Pro.
3) Click once to select the SD card. This will cause the right half of the window to be populated with series of possible actions. The default action displayed is titled 'First Aid'. Locate the 'Verify Disk' button on the bottom right corner. Click once on it and allow some time for it to diagnose your SD card.
4) If the Disk Utility.app reports an error, click to grant it permission to fix it. Once it's done, quit Disk Utility.app
5) Relaunch iPhoto and see if it works.
If this helps, please mark your question as answered. Thanks.