This is on Mac OS X Snow Leopard.

I discovered this morning that a couple of my VirtualBox virtual machines would not start. Double clicking on the name did not work, and the “Start” button remained dim when they were selected.

I ran Disk Utility, selected the hard drive where my VMs were stored, and clicked “Repair Disk Permissions”. After a few minutes of repair work, the virtual machines were unlocked and worked fine.