Two things, if it's a new card and one of the HD cards, some old readers won't see them.

If it worked before and other cards also fail to read (you would need to own two cards), readers go bad.

From a camera? Plug in the USB cable from the camera to the computer and see if the data transfers? Once all the data is on the computer, low level format the card, in the camera, (not on the computer) Sometimes people just copy and delete, copy and delete and the card gets fragmented and acts strange. Mostly gets slower than it should be.

First though, is it a new larger size, HD card? And is this the first time in this reader?