I had this problem today after a power failure.
I had 2 vms and one of them won't boot and was using 100% of CPU and 100% of memory.
Since it was very dificult to solve (at least for me) I want to detail here the steps I've made to fix it, mixing many tutorials.
Best Answer
I had this problem too. This helped me:
Just clicked in the blank area, hit enter, and it started booting.