Just a few hours ago elementary was working fine. I restarted the computer and booted into windows. Now I restarted it again to boot back into eOS and first thing different was that instead the regular login page, where we see the big, beautiful background image and a small window somewhat to the left to log in, there was a black background with a small window right in the middle and inside this window sort of a thumbnail image of the standard elementary background. Also the fonts in this window were totally different than they used to be. I logged in and a totally empty black screen appears. I can see the cursor and move it, but that is all, nothing happens. Now I booted from the live USB I used to install elementary 2 days ago. How can I fix this problem?
The login screen was changed after an update of pantheon-greeter and what you describe now it is the new, normal for all login screen. The empty desktop is an issue for a lot of people, including me. I have NVIDIA graphics and reinstalling the drivers helped me, but sometimes its solved by itself.