After you upgrade to Yosemite; your previous iPhoto version will stop working. This is because the upgrade is not compatible with the older version of iPhoto. Not sure; why Apple has not kept their users informed but since i have had a few customers in a row facing this issue; i thought i should do the guide. In short, to fix the issue with iPhoto; it needs to be upgraded.
Now, You may be able to upgrade to the compatible iPhoto version on Yosemite for FREE but you will need to check this in the App Store. If there are no free upgrades, then you will need to buy it via the app store. BUT if you did had the iPhoto before; then most likely you are eligible for free update.
Please keep in mind that at the time of writing this guide, i had to download the library upgrade as stated in step 4 but this may not be the case in future; as iPhoto may automatically upgrade the library. So if after updating iPhoto; it does this automatically, then you do not need to download the upgrader.
1. First of all, open Finder and locate App Store.
2. Next, open the App Store and in the search bar located on the top right search for iPhoto.
Now, click the small GET button, to begin installing iPhoto. You may be asked to sign in with your Apple ID. After the installation has finished, this status will change to OPEN. Click OPEN and see if you can now view the Photos.
You may be prompted with a pop-up that it needs to prepare library
3. Click Quit to close down the pop-up and download iPhoto Library Upgrader
4. Once downloaded, install it from the downloaded dmg file and then open it.
Click on the continue button, and wait for it to load and begin upgrading the library. Click Launch iPhoto when you see that as an option. Usually, comes after the upgrader has finished it’s job.
Then, open iPhoto and the process should start automatically.