linux mint taskbar disappeared

# In a terminal enter
which xfce4-panel
# This should return:
/usr/bin/xfce4-panel
# (just to make sure it is there)

# Try uninstalling and reinstall of the xfce panel using these steps:
apt remove xfce4-panel
# Ctrl+Alt+Backspace to log out, then log back in. 
# You will have no panel at this point.

# Now kill the controller and delete the config files, Ctrl+Alt+T to open 
# a terminal:
pkill xfconfd; rm -rf ~/.config/xfce4/panel; rm -rf ~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml
# (this should complete without terminal results)

# Finally reinstall the panel. Ctrl+Alt+T to open a terminal:
apt install xfce4-panel
# Again log out and back in. Check the panel