Two initial considerations are
- You should use a USB3 device and port if possible as USB2 may be too slow
- Make sure that the ISO that you used includes support for your graphics card or integrated graphics.
If you would like to try killing X and going to command prompt you could use Ctrl+Alt+F1 or Ctrl+Alt+F2 to switch to a command prompt and restart the login manager.
You can accomplish this by executing
sudo service lightdm stop && sudo startxfce4
sudo xkill -a && sudo startxfce4
If restarting the windowing environment does not work directly, and your hardware is supported, then there is likely something wrong with your Xorg configuration. Get to the command prompt again with Ctrl+Alt+F1.
End the X session again with
sudo service lightdm stop
sudo xkill -a
Backup your current configuration
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.old
Then have Xorg attempt to create a new config file and overwrite the current configuration
sudo X -configure && cp /etc/X11/xorg.conf.new /etc/X11/xorg.conf
Attempt to start XFCE again
If the autoconfig does not work then post the old and new config file here. As well you should install the lspci command as follows.
sudo apt-get update && sudo apt-get install pciutils
Then please run lspci and post the output here. You can save the output as follows.
lspci > lspciOut
Then save it to any writable mounted device.