Help.. VM does not boot up (black screen with progress bar)

[linuxnewbie]

Host: Windows 7 (not sure if this info helps)

I am a newbie with VirtualBox and my OS and hard-ware related knowledge is unfortunately..pretty fundamental.

I installed my first guest (CentOS) a few weeks ago. Used it for a few weeks without any problem. A few days ago, I tried to install a second guest (Ubuntu) but ended up wasn't able to install it successfully due to some read permission to the Ubuntu iso file that I mounted to my Host. Now, when I start my CentOS, it looks like it's starting (there is a bar at the bottom moving, the screen is black). When the progress bar reaches the end, nothing happens - the screen continues to be just black. The guest OS isn't booting up.

Will anyone give me some advice on what I might want to try? Thanks a bunch in advance! It looks like my CentOS virtual machine might have got corrupted. I am really hoping to be able to continue to use it...

Background information (not sure if it helps troubleshooting): I use Daemon Tools Lite to mount the CentOS iso files during the install. There were 2 iso files. If I remember correctly, during the install, the 2nd iso was never really asked for.. I never unmounted the CentOS iso files until when I tried to install my second guest machine Ubuntu (which means the iso files were always mounted when I was using my CentOS). When I tried to install Ubuntu, I tried to unmount my CentOS iso files, and I ran into problem creating virtual drive for the Ubuntu iso file and mounting and unmounting the CentOS iso files.etc. I finally unmounted the CentOS iso files (after a lot of work, including uninstalling/re-installing Daemon Tools Lite, and played around with another mounting software Virtual CloneDrive). And now when I start my CentOS, it doesn't start up.

I don't believe the CentOS iso files need to be mounted when I am using my CentOS virtual machine. Is that correct? I wonder if my CentOS virtual machine was never properly set up even though I could use it and I was using it...
I also tried mounting the CentOS iso files back and then started my CentOS, but it didn't help..

[mpack]

Provide a VM log file covering a problem session of the problem VM. The VM log file is called "VBox.log", and can be found in the "Logs" subfolder of your VM folder. Zip it and attach it here. Make sure the VM is shut down before capturing the log (i.e. I want the complete session from startup to shutdown).