markwmsn wrote: That would happen on an iPhone or iPad, but have you ever heard of that happening on a Mac?
Oops-- kind of knee-jerk response, there. That was all about the "From My Mac" pictures on iDevices, and as you say, it doesn't apply, here.
I wonder if those 50GB of pictures were on iCloud.com before the Optimize choice. One possibility is that Julia_Silnyagina is looking at a completely different Library than she thinks she is.
Hey Julia! Look at the Library chooser to see if you have any Libraries you didn't know about! Close Photos and option-click the Photos icon to bring up the Chooser:

This wil show the path to any other Libraries you have. If you find you have others, you can go there and double click on the Library to open it and see what's there.
I can't imagine why 50 GB of pictures would disappear from a Mac Library just because you turn on Optimize. They might be pictures that can't Optimize because they've lost their connection with Originals, but that doesn't explain why they would disappear locally. So maybe they're just hiding…
Let us know what you find!