Machines are no different than men. Here today, gone tomorrow - in many cases
Try your SD Card Reader on another Windows 7 computer.
When you plug it in, the PC should install the device drivers
and get ready to read , if it is alive.
(If the same brand device was already plugged-in before in that PC, your SD Card Reader should straightaway show as Removable Disks in My Computer.)
If not, replace the reader. (buy one with a detachable cable. In most cases it may only be a defective cable, due to repeated flexing.)