How do I get the IMac to recognize the camera?

I bought the camera and IMac in the Houston area in January. For the first 5 - 6 weeks the camera connected to IMovie on the Mac, and I could download seamlessly. Then, last week the window through which I controlled the downloads started to disappear. At times it would show on the screen for 30 secs, but eventually, I got no window at all. My son also bought an Apple laptop and small camera. When he connects, the movie clips show up on IPhoto, and he can control from there. When I connect, the camera shows on IPhoto but the screen is blank. Nothing shows. I urgently need help, please.

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