I just had a VirtualBox client machine start but the window it was running in was blank. I fiddled around for a while and then waited. Still nothing in the client machine window but blackness.
I looked at the logs held by VirtualBox and there was nothing that looked unusual, although there’s a lot in those logs and I never normally read them so plenty of scope for overlooking something significant.
Then I tried to ssh into the client machine, fully expecting this to fail. It worked!
Then I found this web page and it mentioned Display Memory. Well I knew the display memory had been good the last time I used the VM so I couldn’t see how that was a problem ? But in fact when I went into settings I discovered the Display Memory had been changed to 1MB. My sticky fingers at work I guess ? Regardless I gave it 128MB and closed down the machine through the SSH session, then restarted and, bang !, the machine’s display was back working as it should have been.
Long story short if your Virtualbox machine is blank go and check the Display Memory settings.