iPhoto 11 on Mavericks suddenly can not open library anymore
If you know how to use the info pane in Finder, you can select the iPhoto Library and make sure your user has read+write permissions.

An easy way to test this is to connect a USB or other drive and copy the file to that drive and then click on the library to have iPhoto open it. You might also hold the option key to be sure you select the correct copy of the library.

If that works, you can delete the broken library and copy the fixed one back. Also, if you have a backup, you could restore things from when it worked.

Feel free to comment or edit new info in the post if you have more details to share and this doesn't fix it.