My Device Manager did not show Imaging Devices, so it was impossible to check what is the problem with my webcam.
Here is a good tip which I found on HP's website.
(my laptop is not an HP but the method worked perfectly)
Turn off the computer, unplug power cable, remove the battery, press and hold the power button for 10 seconds. After this, connect everything again and turn on the laptop. Ta-daaa! Webcam works.
But actually it was only a temporary solution. The real problem was that if too many usb-devices were connected to the laptop, it could not reserve enough power to the webcam. So now before I turn on the computer, I unplug my usb-speakers and the external hard drives. That will ensure that the webcam will work.
Another chance to fix this "power-related" issue is the following method: Extremely simple - plug the usb-speaker or the external hard drive to a different usb connection on the laptop. I got 4, but usually every laptop has at least 3 of them.
