This is a very odd problem I've gotten into. When I boot my laptop with a second monitor connected through HDMI, it shows a clone of the image on my laptop (a flashing underscore, that is) but when it arrives at the login screen my TV suddenly says "No Signal" and when I go to Preferences -> Monitor Settings there is no second monitor.
Also, I have another problem. I would love to run XBMC and use the magnificent GUI on my TV (which is why I'm trying to use dual screens in the first place) but when I try to load XBMC it says: XBMC needs hardware accelerated OpenGL rendering. Install an appropriate graphics driver. I don't have any driver install as of now that I'm aware of. Choosing ATI/AMD proprietary FGLRX graphics driver in Preferences -> Additional Drivers didn't resolve the XBMC problem nor the dual monitor problem.
The laptop has ATI Mobility Radeon HD 4330.