This happened to me in x64 Windows 7 after fresh install of OS. It was working before install though too.
Anyway, I think that the OS turns off the driver
for plug and play emul....
Windows did this with mine because the software I installed set the camera setting automatically and made the picture size too big, then it said no camera installed! My camera is a cheap one.
So I right clicked plug and play emulator in device driver, told it to update driver and search automatically for updated driver software...and it did and worked. So that is why I figure it was Windows that turned it off.
And, I was fooling around with the settings in the software...NCH Debut...and reduced the size of the webcam and now the camera works.