how to recover iPhoto library

What version of iPhoto are you running? If it's Version 9.0 or later apply the two fixes below in order as needed:

Fix #1

Launch iPhoto with the Command+Option keys held down and rebuild the library.

Since only one option can be run at a time start with

Option #4 followed by #1 as needed.

Fix #2

Using iPhoto Library Manager to Rebuild Your iPhoto Library

1 - download iPhoto Library Manager and launch.

2 - click on the Add Library button, navigate to your Home/Pictures folder and select your iPhoto Library folder.

3 - Now that the library is listed in the left hand pane of iPLM, click on your library and go to the File ➙ Rebuild Library menu option.

4 - In the next window name the new library and select the location you want it to be placed.

5 - Click on the Create button.

Note: This creates a new library based on the LIbraryData.xml file in the library and will recover Events, Albums, keywords, titles and comments. However, books, calendars, cards and slideshows will be lost. The original library will be left untouched for further attempts at fixing the problem or in case the rebuilt library is not satisfactory.

OT

Fix#1 doesn't work on OS X 10.9 (Maverick) and iPhoto '11 (9.5) 😟

Rather depends on the exact problem you're having. Try fix 2.