It's possible that you accidentaly switched "Activities," which are sort of like alternate desktops that you can run in parallel. If you have a little peanut with some text next to it on your desktop (it might say something like "New Activity") - click on it, choose "Activities," and see if more than one is listed. If so, the other one is probably your original desktop.
To check the keyboard shortcut for switching activities, go to System Settings > Shortcuts and Gestures > Global Keyboard Shorctus and pick "KDE Activity Manager" from the drop down menu. If you did accidentally hit this, you could clear the shortcut from here so it doesn't happen again.
That's just a guess, but I hope that helps.