Since it worked before and does not now, it is probably not a problem on the computer end (unless you did something recently like a system re-install). But be sure to try another USB port, and if you are using a USB hub, try it connected directly to the computer.
Since it does not appear in iTunes or in Windows device manager, it seems like the connection is not working. That could mean the USB cable/dock may have gone bad. Or that the connection point on the iPod shuffle is not working properly.
If you have an Apple Store nearby, you could make a reservation at the Genius Bar. Have them try it with a cable they have, and with any additional testing/maintenance software they may have.