I have installed Ubuntu 16.04 on VirtualBox on a Windows host. Unity doesn't load completely when I login. I am not able to access the Launcher nor am I able to open the terminal. All that I am able to do is open the file explorer, but that too does not show the menu bar. The system looks something like this.
Try installing the VBoxGuestAdditions by going to Devices (while the VM is running) and selecting "Install Guest Additions..." and then installing as if this was from a disk. This seems to be the answer to most graphical issues in VBox.