Autofocus issue

This sounds like a novice question, but I'm no novice; so bare with me.

I've been getting periods of no response from auto focus.

I noticed it most recently after attaching a new Canon Zoom EF 75-300 USM lens. Is there something about this kind of lens that would change the way auto focus works?

I understand the basics of auto focus and the things novices need to learn learn about contrast, etc. and, while it happens sometimes with my slightly shorter lens it wasn't obstinate. The other night a very professional photographer friend grabbed my camera to check out my new lens and auto focus wouldn't respond. Which is to say, it wasn't the subject; he was trying to shoot what I was shooting.

I'm convinced that is has something to do with camera settings; that the shot was too dark or something; but usually prevents the camera from firing; it doesn't affect the focus.

It's a new lens so I haven't r/o malfunction; but i doubt that.

I think it should be noted that were on a night shoot and that may have had something to do with it; but the subject was on a well lit stage.

So the camera isn't trying to focus at all when its dark? If you're in one shot mode, and the scene is too dark, the lens will keep hunting and pecking. If it's the case that it just won't lock on in low light, using an AF assist lamp helps. Check and see if you can AF in daylight situations. Another possibility is that you were in AI Servo mode: you don't get the same AF confirmation then.

Does it "hunt" for focus or just no movement at all?