Destroyed Entries at /dev/sda - boot not possible anymore

[Stefan Lamby]

Hello list.
 
I wanted to install kubuntu 14.04 on a USB-Stick which was mounted as /dev/sdbX
at my productive laptop which uses /dev/sdaX. I choosed all options regarding to
/dev/sdb but after install my laptop was not able to boot anymore since all data
at the beginning of /dev/sda seems to be overwritten. I need to recover and make
it boot again.
 
The laptop (new dell 64 bit) is also running a kubuntu 14.04.02 system.
The setup is:
I have a luks encrypted disk which I am able to encrypt using
cryptsetup open /dev/sda3 kubuntu-vg
On top of it is lvm2 sitting, which I am able to activate using
vgchange -ay
One volume group kubuntu-vg is displayed and I have 2 logical volumes
(kubuntu-vg-root and kubuntu-vg-swap_1). I can mount it via
mount /dev/mapper/kubuntu--vg-root /mnt/
 
When I look at /mnt/boot, I have 2 dirs there: efi and grub

When using fdisk -l I get folling partition layout:
/dev/sda1 2048 1050623 512M EFI System
/dev/sda2 1050624 1550335 244M Linux filesystem
/dev/sda3 1550335 1953523711 930.8G Linux filesystem

 
I am afraid of breaking things more. Also I do not know, how everything is
working.
Are the first sectors of /dev/sda just a pointer to the rest of the booting code
which resides at
/dev/sda2?
 
Please help.
What can I do? What further information could be helpfull?
 
Stefan