[ubuntu] Cannot start guest

I'm running 16.04 (64b) on a Dell Precision M90. I was running without issues Centos 7 guest on virtualbox. The other day, the system crashed and now I cannot get Centos to fully boot. I get the grub menu, select the kernel version and I see the triple progress bar moving very slowly and once it gets to the end, that is the three bars are completely superimposed, then it gets stuck with the screen black. I tried selecting other kernel versions than the last one, but the behaviour is still the same. I really need help on this since I have very important stuff in that virtual machine.

Thanks.