What did you do before you tried restarting the desktop? Can you try to reverse those steps? Like, if you compiled and installed a development version of XFCE or something, go and remove it and install the distro's normal XFCE packages again. Or if you had just updated your distro's packages to newer versions, try finding out how to go back to the previous distro package versions from before the update.

You will have to do that work from a text console. You can get to a console through Ctrl-Alt-F2, F3, F4, F5, F6.

Something else: the command you used has the problem that 'xfwm4' will be running in the foreground in the terminal window where you run it. You can then not close the terminal window without also stopping xfwm4. You can fix that by adding a & to the end of the line, which makes things start in the background:

xfce4-panel -r && xfwm4 --replace &