Can files be restored after OS was updated & RAM added?

So I took my MAC to the "Best" Buy Geek Squad today to add ram and update the OS. They said NOTHING would be deleted, no need to back anything up. Just set my computer up now and EVERYTHING is gone. EVERYTHING. Every single picture and video of my baby since he was born...GONE. I am SO upset. Please, someone tell me there is a way to recover these files!! I could die right now...the pics and videos are most important but I had SO MUCH important stuff on this computer. I called the Geek Squad 1-800 # and they could give a f, of course. They said that the local store SHOULD have backed up my files but if they didn't OH WELL. If I use Time Machine is it going to delete the new operating system? Someone please help. Let me know if you need more info. I'm not the techiest chick and I'm also completely frazzled and upset...