help!computer doesn't see camera

Up until last week, allI had to do was plug my PowerShot SD770IS into the computer to download photos. Now the computer cannot detect the camera. What could be wrong?

What computer and what operating system, for starters?

did you change any of your computer settings recently?

The camera is a Canon PowerShot Sd770IS. The computer is running Vista. When I plug in the camera, the computer doesn't see it.

Plug the camera in another USB port and see if it works especially if you are using the front USB ports, in which case, try plugging it in to the rear USB ports.

It could be anything at this point. Try these things in order....

1. Try the camera on an other computer + try another USB device on your computer + try your device on other USB ports on your computer. If it works in another port you have a dead port. If it works on another computer but not yours then, then see below if no then your camera's USB plug or the USB wire are bad. Change wire and repeat - if it works then go to #2 if not then your camera USB port is bad. Solution: Use a card reader.
2. If it worked on another computer then the USB (drivers) are probably corrupt.
A. Uninstall camera TWAIN drivers then us CCleaner to cleanup Registry
B. Reinstall TWAIN drivers and reboot. Does it work? No, then continue.
3. If it is not the TWAIN driver's then it may be the USB drivers.
A. Go to Computer > Properties > Device Manager > Portable Devices and erase your camera if you see it - reboot and retry. If it is not there then keep going.
B. Install the chipset drivers for your particular motherboard (use your manufactures not Intel's UNLESS you actually have an Intel motherboard). Reboot. Still no luck? Keep going.
C. Go to Computer > Properties > Device Manager > Universal Serial Bus controllers and uninstall everything that says "USB Root HUB", "USB Generic HUB", "USB Mass Storage Device" but do NOT delete anything that says "Controller" (basically, delete all the HUB's and leave all else alone). Some of your devices may stop working (Keyboard/Mouse) but they'll call come back after a fresh reboot. Still not luck? Keep going...
D. Find your motherboard's firmware and install it. Reinstall your motherboard's chipset drivers. Still no joy? I'm out of ideas

Very strange! I plugged the camera in again this morning and up came the photos. Didn't change anything.