unit – Can not start after a lubuntu-desktop environment

I have two problems.

Problem 1 : My old laptop runs Ubuntu 18.04 (Unity). Yesterday I was trying other office environments like Gnome and I wanted to try Lubuntu. So I installed it with the help of the following command:

sudo apt-get install lubuntu-desktop

When it was installed, everything worked well. But after a reboot – after showing the sign "Lubuntu", there is this black screen. (Therefore, I can not connect)
All I can do, ctrl + alt + f3 and enter the console.
So after searching the internet, I uninstalled lubuntu-desktop using

sudo apt-get remove lxde

sudo apt-get remove lubuntu-desktop

(or something like this after logging into the console.)

I've tried reinstalling the desktop, lightdm and nvidia ubuntu drivers but still nothing.

How can I put it back to work?
How can I safely install an office environment? I remember having a hard time with Gnome too. : S

Problem 2:
This old laptop can boot to Ubuntu 18.04, 15.04, Archlinux and Windows 10.
The base folder on Ubuntu 18.04 is encrypted with ecryptfs. (Or maybe even LUKS? I'm not too sure)
How to access it from ubuntu 15.04?