Two possibilities. You may be opening the Render Engine instead of AE. Please carefully check the app name. If you are opening Adobe After Effects and you still get the render engine then you have to follow the instructions in the following link and remove any file named ae_render_only_node.txt from your Documents folder.
The other possibility is that you have simply fouled up a workspace. Try Resetting by spinning down the Workspace list or by going to Window>Workspace>Reset