apt - Recover data when Ubuntu 14 crushed

My situation is the following. I had Ubuntu 14 installed on this machine, which stopped responding. (Precisely: After a suspension it refused my password for a few times and it refused to shut down. After forcing the shut down I restarted it but there was no OS on the machine anymore.)

I burned Ubuntu 16.04 LTS on a USB-stick and I'm now in the "try mode", the reason I'm not immediately installing is that I would like to recover the data I had on the previous system. But, I don't seem able to do so. Indeed:

1. My hard drive was partitioned into

   
   
   
   
   
   • /dev/sda and /dev/sdb, but Disk utility only sees /dev/sda.
   
   



2. On the other hand GParted sees both the partitions, but when I try to recover data it says that I need gpart. If I try

   
   
   
   
   
   • sudo apt-get install gpart
   
   

the outcome is

Reading package lists... Done Building dependency tree Reading
state information... Done Package gpart is not available, but is
referred to by another package. This may mean that the package is
missing, has been obsoleted, or is only available from another source

E: Package 'gpart' has no installation candidate

(I already upgraded and updated apt-get)

3. I then tried to move to Testdisk, but if I try to install it

   
   
   
   
   
   • sudo apt-get install testdisk
   
   
   

the outcome is

Reading package lists... Done

Building dependency tree

Reading state information... Done

E: Unable to locate package testdisk

4. And if I now try to update apt-get

   
   
   
   
   
   • sudo apt-get update
   
   
   

the outcome is

Ign:1 cdrom://Ubuntu 16.04.1 LTS Xenial Xerus - Release amd64 (20160719) xenial InRelease

Hit:2 cdrom://Ubuntu 16.04.1 LTS Xenial Xerus - Release amd64 (20160719) xenial Release

Hit:4 http://archive.ubuntu.com/ubuntu xenial InRelease

Get:5 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]

Get:6 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]

Fetched 204 kB in 0s (464 kB/s)

** (appstreamcli:31465): CRITICAL **: Error while moving old database out of the way.

AppStream cache update failed.

Reading package lists... Done

Overall I feel pretty much stuck. Is there a way I can install gpart or testdisk? Is it possible that a malicious virus already deleted all my data after kicking me out of the way? What could be the causes why apt-get is not working?

Thank you very much for your time!