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.