To run that command open up a terminal window and type the following
line ...

sudo /etc/init.d/vboxdrv

... then press [Enter]. It will ask your password then run the
command as root. Hopefully this will get your VM working again but I
have heard that upgrading to VBox 4 can mean re-creating VM's in it
from scratch.

Having switched to VBox 4 myself ( using clean installs of VM's ) I
like the changes made and find it more solid overall. Had a little
bother with getting 11.10 Beta 2 going until I used a VHD instead of
the VDI option - now running well :-)