Assuming you're putting an SD card into the SD card slot there's only a handful of possibilities:
1) The SD card isn't supported by the card reader. High capacity SD cards may have come out later than the reader/laptop and aren't supported.
2) The OS doesn't have drivers for the SD card reader. You put a card in, the reader might know that but the OS has no clue because the reader isn't communicating with the OS. All you'd need is the drivers which may be on the manufacturer's website. Likely the device shows up as an unknown USB device with a yellow triangle.
3) The card reader is disabled in the BIOS. Here the SD card reader doesn't even show up as an unknown device since it's effectively not installed. Turning it on in the...