Without more information we can only guess. However, I think that this is likely due to the host OS having a kernel upgrade and the VirtualBox modules not being created and loaded. So, the first thing I would do is ensure that the current Linux headers package is installed:
apt-get install linux-headers-$(uname -r)
Then it may be necessary to reinstall virtualbox.
Edited to be more up to date, changing "``" for "$()".