The easiest solution to bringing back the XFCE panel is to either right-click the desktop (not any icon on it) to bring-up the desktop menu, and select "terminal" from "Accessories" category of the "Applications" submenu. One can alternatively press the "Alt" button, continue to hold it down, and press "F2"; the convention for shorthand regarding the pressing of key combinations of what I just wrote is "Alt+F2", meaning "Press 'Alt', and then while still holding 'Alt', press 'F2'"; Alt+F2 will initiate a little "Run Program" dialogue box into which is typed
and then press "Enter" or push the onscreen "Run" button to start "Terminal".
When "Terminal" has started, first type the command
killall xfce4-paneland then press enter. After that type
xfce4-paneland then press enter. To simplify the whole process, if after several repetetitions of this process as needed the output after the "killall xfce4-panel" command is something like "no process killed" or "no such process found", then the panel is likely not just disappearing, but as a process is altogether shutting off or dying (which may be form an internal bug, or some other conflict with an external process), and if this is the case the user may thereafter skip the terminal altogether by simply pressing Alt+F2, and into that box entering
xfce4-panelafter which "Enter" or the onscreen "Run" button is pressed.
Terms for which people might search include "XFCE panel bar", "XFCE Start Button", "XFCE launcher", "XFCE launcher bar" inter alia(def.).