I've attached a HDMI monitor to my Pi running Raspbian. In the past external displays have worked fine.
Recently, when I connect my Pi to a HDMI monitor I get the usual bootup screens and the bootup log scroll. I can even access NOOBS and configure it there. But as soon as the bootup scroll finishes, I'm stuck at a screen with a blinking "_" in the corner. The Pi is otherwise working fine, since I can log in via SSH.
The only hardware change I have made is to add a 4D systems 2.5inch GPIO touchscreen. I can't remember if the HDMI output worked after installing the GPIO display.