Linux boots into memtest86

Today I had the problem, that my ubuntu instance was booting automatically into the memtest86 grub entry. This was mainly because that was the only grub entry that remained after a linux image update.

The simplest way to fix this problem was using a live CD and the following commands:
1) boot into live CD

2) create a directory to mount the old linux on your hdd

sudo mkdir /mnt/temp

3) mount it

sudo mount /dev/sda1 /mnt/tempfor i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt/temp$i; done

4) copy the resolve.conf into the original ubuntu so that the apt-get will work in future commands

sudo cp /etc/resolv.conf /mnt/temp/etc/resolv.conf

5) change the root directory to the newly created mount point

sudo chroot /mnt/temp

6) now check if the kernel is already on your system. If not just install it

apt-get install linux

6.1) do some cleaning in your system

apt-get autoclean
apt-get clean
apt-get update
apt-get upgrade
apt-get install -f
dpkg --configure -a

7) if this still not works try to reinstall grub

apt-get purge grub grub-pc grub-common
apt-get install grub-common grub-pc
update-grub

8) umount the whole stuff

for i in /dev/pts /dev /proc /sys / ; do sudo umount /mnt/temp$i ; done

9) restart the system

Now it should boot into the actual linux kernel and your system is back!