I have experienced two cases where lxc containers stop working or new containers never work because they cannot join the network. My LTS container started, but without network, there were a lot of things I couldn't do in it. In the case of Juju, I couldn't deploy new local services. The unit agent status was stuck in PENDING.

You can verify the network is broken by fancy listing the lxc containers you started either by lxc-start or juju deploy:

sudo lxc-ls --fancy

NAME        STATE   IPV4 IPV6 AUTOSTART
-------------------------------------------
<container> RUNNING -    -    NO

This shows that the running container doesn't have IPV4 or IPV6 networks.

For each existing container that needs fixing, you need to install the new dhcp packages, but without a network, you cannot do it from a running container. Instead chroot can be used to update the container's root file system.

cd /var/lib/lxc/<container>/rootfs/
sudo chroot ./
sudo apt-get update
apt-get install isc-dhcp-common isc-dhcp-client

To ensure that new containers work, you need to clear the lxc cache. The lxc image cache was built when you created or deployed your first container. The images there are probably more than a few months old.

sudo ls -lh /var/cache/lxc/precise/
sudo ls -lh /var/cache/lxc/cloud-precise/
sudo rm -r /var/cache/lxc/*

Lxc will get the new images the next time it needs to create a container. The first call will be slower to complete since it will build a fresh cache.