Usually if you log out, then log back in again after choosing a different session at the login screen, it will go back to normal. At the login screen, look for a button that says "Session," or "Session Manager," or something similar, and you should be able to choose to start XFCE again. It will probably ask you if you want to set XFCE as your default session. Click yes, and it will start every time you login.
Often times when a new desktop environment is installed, it thinks it should be the default, and you have to change it back.
Good luck. Don't give up.