You're going to want to boot up into text mode, rather than graphic mode (since you're getting the whole "screen turns off" problem) Since you're using GRUB scroll down to the option to select Linux, as you normally would. But, instead of hitting [ENTER], hit [e]. At the prompt hit  and then hit [ENTER]. Now, hit [b] to boot as you normally would, although this time, instead of getting the usual "Hey my screen's not working" you'll get something like this
If you enter your username and then your password, you'll be on your computer, just in text mode (Or runlevel 3). So, this is where you would enter the commands as tricky_linux has told you. Hope that clears things up!