There are a number of problems that can arise with hardware on a computer. Conflicts among a hardware's driver and other programs on the computer might cause a sound card to function incorrectly or not function at all. If your computer doesn't recognize a new sound card, it can't access it at all. Luckily, if there's some kind of problem like this with your sound devices, it's generally pretty easy to determine.
Open the "Control Panel" located in the "Start" menu.
Click on "System," and open "Device Manager." On some versions of Windows, the "System" menu is called "System and Security."
Click on the arrow next to "Sound, Video, and Game Controllers." This has a list of sound and video devices on your computer, including your sound card.
Select the device you want to check, and right-click on it. Select "Properties." This brings up another window, which says, "This device is working properly" if there are no issues with the selected device, or, "This device is not working properly" if there's some sort of problem with it.
A common problem is often with a device's driver. These types of problems are usually fixed by uninstalling the driver, which you can do in the "Properties" window you opened to see if a device was working correctly, and reinstalling it. Depending on the device, you may need to access the manufacturer's website for an updated driver, or you can simply uninstall it, and allow your computer to detect the device again.
If some devices are missing from the "Sound, Video, and Game Controllers" category, it usually means your computer isn't detecting the device. Look on the manufacturer's website for the most up-to-date driver for your device and install it.