Normally this is kind of problem is user error. If you are new please start here: Basic Workflow
I really tried to figure out what you were doing because we don't have any details on your project or render settings. The video may actually be OK because your Media Player does not support Alpha Channels, or you could have something going on with your layers. Drop the rendered video into AE and put it in a comp to see if there is anything there and reveal it's properties. If there is nothing there then you should drop a white solid below all of your layers to give the video a background and then render it again. If you want a transparent background then you'll have to choose Lossless With Alpha from the presets in the Output Module. If neither of these suggestions work then you have to tell us all of the details of your project and give us a screenshot showing what is going on with the layers.
Placing a white solid background solved the issue.
Rendered video here (and my first animation):