This page has pictures of all of the different devices so you can tell what generation you have:
The remote is only for the new Chromecast with Google TV device. That is the oblong one. The remote won't work with any other device.
You can order a new remote here:
Or, if you have a compatible Android phone or tablet, you may be able to use the new virtual remote:
For all the other generations (and, if you want, even for Chromecast with Google TV) you control it using the apps on your phone or tablet.