Hi,
1. Which is the make and model of the external webcam and the computer?
2. Is the issue specific to a program that you are trying to video chat?
Try connecting the webcam to a different USB port.
Check if the webcam is getting detected in Device Manager.
To open Device Manager:
a. Click on start button.
b. Type devmgmt.msc in the start search box and press enter.
Follow these steps:
Method 1: Run the Fixit
Hardware devices are not working or are not detected in Windows “ : http://support.microsoft.com/mats/hardware_device_problems
Method 2: I would suggest you to update the drivers for the webcam and check if it works.
Follow these steps mentioned below to update the drivers manually:
a. Open Device Manager by clicking the Start button, clicking Control Panel, clicking System and Security, and then, under System, clicking Device Manager.
If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
b. In the list of hardware categories, find your webcam, and then double-click the device name.
c. Click the Driver tab, click Update Driver, and then follow the instructions. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
Refer this link for help:
http://windows.microsoft.com/en-us/windows7/Update-a-driver-for-hardware-that-isnt-working-properly
Refer this article:
Seeing is believinghttp://windows.microsoft.com/en-US/windows7/Seeing-is-believing-Use-a-webcam-to-bring-your-instant-messaging-to-life