VirtualBox requires that its kernel module is recompiled every time the Linux kernel itself is updated (which happens surprisingly often with Ubuntu). This can be done using

/etc/init.d/vboxdrv setup

If this fails, check the log file at the location given for the reason. The most common reason is that the kernel source is not installed on the machine. To install the source, do

apt-get install linux-headers-<kernel_version> 

then re-run the vboxdrv setup command.

Test the compilation is successful by executing