I have Ubuntu 12.04 with btrfs as my main FS. Once Windows was install Ubuntu vanished as usual. Took some playing but here is my fix.
The guts are..
sudo mount -t btrfs -o subvol=@ /dev/sda6 /mnt/
sudo mount --bind /dev /mnt/dev/
sudo mount --bind /proc /mnt/proc
sudo chroot /mnt/
grub-install --recheck /dev/sda
The key bit is the mount line. If you see in your mount directory @ and @home something has not worked and it's mounted the raw thing not the correct subvolume.