Liat:
Welcome to the Apple Discussions. Assuming you have sufficient free space on your HD consider this: did this begin after upgrading to Leopard? If so then it's a problem with the differences between Tiger and Leopard and privileges. I had to do the following go fix my library:
1 - open Terminal and type “sudo chmod -R -N “. Note the space after the N.
2 - drag the iPhoto library into the Terminal window and hit the Return key.
3 - enter your admin password when prompted and hit the Return key again.
That should fix it for you.
Thomas is correct about free space. It is desirable to have a minimum of 15GB free for optimal system and application performance. The system can run with as low a 5GB or even a little less but not without taking a big performance hit. If you accidently let the free space get down to 500 KB or less, you can get to a point where your can't boot. A few have gotten there.
Happy New Year
TIP:
For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've created an Automator workflow application (requires Tiger), iPhoto dB File Backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. It's compatible with iPhoto 08 libraries and Leopard. iPhoto does not have to be closed to run the application, just idle. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.