Think of your computer's hard drive as a temporary solution for any files. It's not a matter of if things will disappear, but when. Regular readers can guess what I'm going to say next...
I’m very sorry to hear this.
From your description, I’m not exactly sure what happened. I don’t necessarily have a specific solution to help you recover what you’ve lost, but I have a variety of thoughts on this kind of situation.
If I’ve said it before, I’ll say it again. I say it so often that people get annoyed with me.
And yet, I cannot stress enough the importance of backing up.
Many people set themselves up for disaster by keeping photos, files, and other important data in exactly and only one place. If photos are only on your machine or your camera, there is a high probability that you would have lost them anyway. If the loss wasn’t because of a Windows upgrade, it’s possible that you would have lost them because of disk failure, a power glitch, or a software problem.
There are many ways that files can disappear permanently.
This sounds like what happened to you. A backup of some sort would have saved you.
Always back up before you upgrade
The other problem is that you don’t say what you upgraded from or what options you selected when you did upgrade.
An upgrade from Windows XP to Windows 8 simply does not save all files.
Part of the process for upgrading is to back up first and then restore your data files after the upgrade is complete. In fact, that’s the most recommended option for upgrading from any prior version of Windows to a new one. It’s just too easy for things to get messed up.
That’s why I almost always say, “Back up first, and then do the upgrade.”
If you upgraded from other versions of Windows, there are options that will actually preserve your data. But as you don’t say what or how your upgrade was performed, I can only guess that this was the cause of the loss.
Check with Picasa
Just in case, check with Picasa online. In addition to being a desktop photo management application, Picasa is also Google’s photo sharing service. It’s possible that some of your photos were preserved online. Again, this depends on the settings and choices that you made. I’m not terribly hopeful, but it’s worth a look.
Photos are one of those things that simply cannot be regenerated once they’ve been lost. At this point, all that I can really recommend – not just to you, but to anybody reading this article – is this: if you aren’t doing so already, please start backing up.