Entire photo albums missing from Photos app on Mac

I have a 15-inch mid-2009 MacBook Pro. Up until about 4 days ago, I was running Mavericks. My entire photo library (almost 6,000 photos) was housed in iPhoto, all neat and organized into several photo albums. I decided I wanted to take advantage of iCloud syncing, so I updated my MacBook to Yosemite, which automaticlly migrated my iPhoto library to Photos. I also upgraded my iCloud storage to 200GB, to more than accomodate my ever growing photo library.
The problem I've encountered is this: Last night, I noticed that a handful of entire photo albums disappeared altogether, and I have no idea where they went. After messing around trying to figure it out, I realized that Photos had stopped uploading photos because my hard drive was full. So, to free up about 60GB of space (which was more than needed to complete the uploading of the rest of the photos), I moved my iTunes library to an external hard drive. I then noticed that the blue progress bar on Photos>Preferences>iCloud resumed uploading, however, when it finished and said Updated just now, the missing albums still were not visible. They are still missing, and I'm freaking out.

Anyone know what the heck happened??