File recovery after reinstallation

First of all, you should stop using the disk, to avoid overwriting the text (if that has not already happened...). From a live system, you can 'grep -a' the (unmounted) partition where the deleted file was probably written (something like /dev/sda1) for part of the text in that file. For instance, if it contains "magic banana" but you are unsure of case (use of option -i), and if there are no more than 10 lines before "magic banana", and 100 after, then the following command should output (here redirected to a file named "out") your file (and possibly much more, if you used "magic banana" in many other files: the searched part of the text had better be specific):
$ grep -aiB 10 -A 100 'magic banana' /dev/sda1 > out

Do you have a dedicated /home partition? Have you formatted the /home partition during installation?

I've tried grep with USB live version but didn't find what I need (tried a few words from this document I remember but with no success - just some files I already created with this instance).
Dedicated home partition? I don't know what it means.
I installed the new instance on top of this one (just clicked next->next->next, basically) I just didn't checked the "format whole empty space" option when choosing the destination partition.