Hi @Aga,
if you ever need to do a new (re-) installation, follow this guide and it might save you a lot of headache if you haven't already done so, and/or don't know about it:
If you can get into "Recovery Mode", do the following:
To remove a package using the command line in "Recovery Mode":
Start in recovery mode (a network cable connection is required!) and first activate "Network" and then click on "root", then type the remove command (write it down on paper first).
To remove a package, just use the normal remove command like the following:
sudo apt-get remove firefox
Firefox is only an example, type the name of the package you want to remove replacing "firefox with the real package name!.
A real good example would be graphics card drivers which may be causing a problem, lets use an Nvidia package as an example:
sudo apt-get remove nvidia-304
which will remove the Nvidia 304 drivers package, restart with Ctrl + alt + Del, don't click on "Resume"!.