Here are a few things to try: 1.) Ensure the card reader is plugged-in and working properly by testing it with another card that you know works. 2.) If the SD card doesn't have data on it, try formatting it. 3.) It could be that you need to install a special driver to get the reader and/or card working.
Go to Control Panel > System > Device Manager and click on Scan for Hardware Changes. The window should refresh with any new hardware that has been found.
You should search for a driver with "Windows Update".
If no drivers are available, try looking up Your computer manufactures website.
Often they have specific drivers for Your PC.
One reason can be that your card reader does not support SDHC cards. HC stands for high capacity. Check the label of your card and the manual or specs for your reader. Test the card with other readers or devices for example a camera. That way you can make sure that the card is not the problem itself.