Ubuntu 12.04.2 server amd64 on Virtualbox

Hello,
I could not find the appropriate forum for my problem to boot Ubuntu 12.04.2 server amd64 on Oracle Virtualbox(4.3.0 BETA1) within Win 7, so I posted here to start.
For some reason Ubuntu could not boot anymore, and I am looking any suggestion before I re-install it as I do not want to erase all the old stuff within it.
Symptoms: Freezes at boot up.
Tried to fix with startup iso image, blank screen after I select language screen(i.e. the first screen for rescue broken system).
Any suggestion is greatly appreciated!

YT