VirtualBox log files is one of the great tools to diagnose an issue in your VirtualBox. It will always create a log file which reflects the lifecycle of your virtual machine.
VirtualBox log files is located in user/per-vm directory. Depending on the OS:
On Windows, %HOMEDRIVE%%HOMEPATH%\.VirtualBox\Machines\\Logs
E.g. C:\Documents and Settings\Username\.VirtualBox\Machines\\Logs\vbox. log
On Mac OS X, this is $HOME/Library/VirtualBox/Machines//Logs
On Unix-like systems (Linux, Solaris), $HOME/.VirtualBox/Machines//Logs
The log files are rotated such that the most recent is always called vbox.log and older ones are vbox.log.. These log files contain lots of information about the capabilities of both the host and the guest vm and should be provided whenever reporting issues with VirtualBox.