Sometimes the dock and top bar do not appear. What to do?
I usually just Ctrl+Alt+F2 login to command line and reboot. It then goes away.
I was also having this problem intermittently, but it was due to a different bug and this fix solved it for me:
sudo mv /etc/xdg/autostart/at-spi-dbus-bus.desktop /etc/xdg/autostart/at-spi-dbus-bus.disabled
If you have an Intel Graphics card that's a known bug and it should be solved by now. Try updating your system (open AppCentre, go to Updates and press Update All).
Still happening randomly at boot.
A solution faster than rebooting is to switch to another console and restart lightdm with
sudo service lightdm restart
Do you have a logitech wireless mouse or keyboard?
If so, Try unplugging the receiver and see if the wingpanel comes back