Black screen in Ubuntu in Virtual Box

I had this same exact issue. I solved it by shutting down, going to my virtual machine settings and selecting display. I changed the graphics controller to VBoxVGA, yours might be different, just interchange the available options. I hope this helps

My display was already set to VBoxSVGA. I got the solution from Ubuntu 18.04 LTS on Virtualbox boots up, but black login screen which was to change the Video Memory from 16MB to 64MB.

I am running VirtualBox 6.0.14 on host Ubuntu 18.04.2 (Dell laptop). My Debian 9 VM was starting with a completely black, blank screen.

For me, the issue was resolved by changing to:

Display → Graphics Controller → VBoxSVGA

I am running VirtualBox 5.2.6 on a Mac OS host. I had to change the number of processors to 2 (Settings->System->Processor) to get the login screen to show up. The default appears to be 1 processor (at least for this version of VirtualBox).

Very Late but simply disabled 3d acceleration and VMSVGA as the graphic controller and it worked. The cause of my issue was running out of cd memory + subsequently adding more but then the VM wouldn't open after the logging in(black screen).

also changing boot order to CD worked on a different occasion