My iPhoto won't work at all after downloading iOS yosemite.Any help is appreciated.

Yosemite is not an iOS system, it's a OS X system. There are no previous versions of iPhoto that will work with Yosemite. You'll need to go to the App Store and download iPhoto 9.6. It should be a free download. It will be located under Purchases. If it shows a price click on it anyway and it should begin downloading for free.

I updated to Yosemite. Went to app store and clicked to update iPhoto. When I go to iPhoto it says iPhoto library needs to be updated to work with this version.

If I do the update will I lose my library of pictures?

When I go to iPhoto it says iPhoto library needs to be updated to work with this version.

If I do the update will I lose my library of pictures?

The library update is necessary, that iPhoto 9.6 can work with your iPhoto Library. It is a routine procedure after updating the iPhoto application.

But better make a backup copy of your iPhoto Library, as a precaution, just in case. Your iPhoto Library is by default in your Pictures folder. make a copy on an external drive, if you do not have a backup.

I would be very careful, I selected the upgrade photos and now I have a 200GB library file but no way to access the photos, it appears to be something to do with the 10.10.2 upgrade and the use of an external Thunderbolt drive to store my photos on, but I am nearing desperation in trying to get back my years of photos (last effective backup - despite having time machine turned on) - appears to be 2012.

This thread seems to have the most information

Yosemite, iPhoto 9.6 upgrade, large photo library is not recognised.

But it is outrageous that a 'minor' upgrade that was not meant to be about iPhoto could have such drastic outcomes. It must be relating to the file service changes they were doing or the work they were preparing for the new photo app - either way, not happy!

You have posted this exact same post in three different topics and in none do you give any details about your situation.

What system and iPhoto versions did you have before the update to 10.10.2?

What version of iPhoto are you running now?

How is the external HD formatted?

Are you running a "managed" or "referenced" library?

How much free space on the EHD and boot drive?

What fixes have you tried so far?

If you don't give us something to work with we can't possibly help.