If USB, unplug and replug and see if Windows will see it as new device. If internal card reader, go to My Computer and see if it shows us with a drive letter. If so, take card out and reinsert to see if it recognizes it. If not, and it shows a drive letter, click on the drive and see if it opens the disk.
Could be that software that opens the drive upon inserting SD disk can be corrupted or machine needs a reboot.
Hope this helps.