How to troubleshoot booting to a black screen on VirtualBox

I had this problem and my solution was simple.

Somehow, my VirtualBox configuration for the VM (Ubuntu) was changed after a VirtualBox upgrade. My guest VM was Ubuntu 64 bits and it was configured as 32-bit version. To change:

1. Open VirtualBox
2. Select your virtual machine and click on Configurations
3. Go to General
4. Change the version to Ubuntu (64-bit)

This is how I troubleshoot a non-starting Ubuntu in Virtual Box (assuming optimal settings for a virtual machine were already made):

1. Make sure you had installed the most recent version of Virtual Box.

2. Try a reboot by entering a TTY terminal (Host + F1)

   sudo reboot
   

3. Update the guest OS from TTY (Host + F1):

   sudo apt update && sudo apt dist-upgrade
   

4. Install guest additions from command line.

5. Reboot the guest after installing the guest additions.

6. Start lightdm manually from a TTY (Host + F1) with

   sudo lightdm
   

7. Try several reboots of the VM. Due to timing issues it may help eventually.

8. Try a reboot of the host.
9. Check forums and virtualbox.org for known bugs.

For me, none of the previous answers worked. I solved this problem by disabling 3d acceleration on the vm settings page. This is quite odd since I've been using 3D acceleration flawlessly for months on that VM.