I have had a Canon T1i for about 4 years now and I've noticed that it won't take pictures in low light even when I'm in manual mode. I've taken some pictures using time exposure in "bulb" and even then sometimes it won't release the shutter. Any ideas why?
The manual (M) mode only means the light metering is manual. Focusing is still automatic unless you toggle the MF/AF switch on the lens. In your case, there's simply too little light available for the AF system to find focus, and by default the camera will refuse to take a picture in this situation.