hi
i'm using Debian 8 on a Sony NW21SF laptop. A new problem came up recently, when i try to login, it dropes me back to the login screen. I can use the X-window-system if i login as root on a tty terminal and type "startx". Any suggestions?

thank you

I have to mention that if you try to log in as root from the graphical login screen you will see this behavior. Thet's probably not what you're doing but felt it worth typing for completeness.
Secondly, which DE or window manager do you use and which login manager? If default it will be GNOME DE with gdm3 so it's worth selecting "fallback mode", if present, or ensuring Wayland isn't selected (or is if it isn't).
If none of that helps I would be tempted to rename (using mv on the command line) the hidden directories which hold the setting for your DE -- sorry, I am not at my machine to find them but a google should produce them. It could be that the update installed DE components not compatible with your settings or that your settings are corrupt in some way.
You could even just try running the apt-get update and upgrade cycle again and see whether it fixes itself.

i can't find this option, i don't really understand what i'm doing. Is this a config file?

thanks

At the GRUB menu, highlight the Debian 8 entry and press "e" then add this to the end of the line that starts with "linux":
Then press <Ctrl>+x to boot the modified entry.

This will boot up to a console (TTY, text only) log in after which you can try `startx` as a regular user.

Does the desktop start then?

Which desktop environment are you using?

Do not use your graphical desktop as root.

Assuming this is a default Debian install then, when you see the graphical login, there should be a little "cog" icon which you press to select session type before you log in.
To get to the command line or shell as it is generally called from that same screen you can press Ctrl+Alt+F2 (or another of the first 7 F keys butone will put you. Back at the graphical login.

It sounds more like a permissions problem. Make sure that you are the owner of the home directory. Change it with (substitute user with your username).

If this does not help, remove (or change the names if you intend to keep them) these files from your home directory:
.xinitrc
.xsession
.Xauthority

That should suffice.