OK, you need to edit a line at the GRUB menu screen at bootup, instructions here for GRUB2 (Ubuntu):
https://help.ubuntu.com/community/Grub2 ... ing%20Boot
and add a space after quiet splash then the i915 modeset=0 (or nomodeset)
so the end reads:
quiet splash i915 modeset=0
It should be *similar* in Debian and/or GRUB legacy. (though it might be the Esc key rather than Shift)
remember, this will NOT survive a reboot, so you will have to add it permanently to GRUB after booting and update-grub
Which version of Debian ? .. or GRUB ?
Or you could use a working LiveCD (or Recovery Mode root prompt) to navigate (on you hard drive) to:
and change its contents from:
options i915 modeset=1
options i915 modeset=0
Save the file, and reboot.
as explained here: