How to make Ubuntu Server laptop work with external monitor only

An Ubuntu Server laptop supports 1280×800 resolution for its internal display, but its Intel GMA4500MHD GPU can support 1920×1080. I plugged in a VGA external monitor capable of this desired resolution, and my goal is to make this laptop use solely the external monitor, and at 1920×1080.

Now I’m being faced with two issues. First of all, running xrandr -q returns a Can't open display message. Second, meddling with /etc/default/grub (after having read other AU threds) has not produced any result so far. Please advise — thanks in advance.