All 18,000 of my photos/videos have disappeared. Please help!

Kind of logical question, have you checked your recylce bin?

I upgraded to Yosemite over the holidays.

What happened directly after you upgraded to Yosemite, when you updated iphoto? With Yosemite you need iPhoto 9.6, so you must have updated or upgraded the iPhoto Library at some point. Did you?

How large is your iPhoto Library? If it is larger than a few MBs, all your photos are probably still inside.

Select your iPhoto Library in the Finder and ctrl-click or right-click it. Use the command "Show Package Contents" from the pop-up panel.

What do you see in the Finder window, that will open? Are there folders "Masters" and "Old Masters"? How large are these folders?

Restore the backup copy of your iPhoto Library from the backup made just prior to your upgrading to Yosemite.

I am running iPhoto 9.6 although I never upgraded.

You must have. iPhoto 9.6 requires Yosemite, so you could not have been running it previously. You may want to check your System Preferences > App Store setting. Perhaps you have "Automatic Updates" enabled.

And again - restore the backup from just before this occurred - that is why you must always have a good current backup - bad things happen and a backups the protection against that

LN

ok, any way to get my 18,000 photos back?

There are two possible scenarios - I can't tell yet, which it is, since you did not tell us much about your iPhoto library. Where did you keep it? In the Pictures folder or on an external drive? And which version of iPhoto created it? The answers to both questions might explain, why iPhoto could not open the library.

1. Scenario 1: iPhoto created a new, empty iPhoto Library, because it could not open your iPhoto library (it may be stored on an unsupported disk, the version may have been to old (iPhoto 7.x or older), or a permissions problem or library corruption). In that case your iPhoto library will be very small, less tha a GB.
2. Scenario ": iPhoto tried to upgrade the library, but made mess of it and corrupted the library because of a bug. Then your iPhoto Library will be very large, several GB, when you view it in the Finder.

What ist it? Look into the Pictures folder and check the size of the iPhoto Library, and also look inside, as I described here:

Select your iPhoto Library in the Finder and ctrl-click or right-click it. Use the command "Show Package Contents" from the pop-up panel.

What do you see in the Finder window, that will open? Are there folders "Masters" and "Old Masters"? How large are these folders?

I don't think you opened the iPhoto Library package, when you answered my last question.

my iPhoto library is small 14.7mb. No masters or old masters.

That is just the size of your two photos.

So iPhoto opened a new iPhoto library because your old library could not be found or opened. Did you wipe your disk completely, when you upgraded to Yosemite?

What kind of backup do you have? A Time Machine backup?

Here is a short support document: iPhoto '11: Restoring from Time Machine with iPhoto '11 (9.2 or later) and OS X Lion 10.7.2 (or later)

And this document explains in more detail how to restore from Time Machine (scroll down to the lower half):

Mac Basics: Time Machine backs up your Mac - Apple Support

léonie,

Did installing Yosemite erase his original iPhoto library or just create a new library? If he didn't wipe his drive, is it not possible that his old library is still sitting there?