Q: El Capitan downloaded but it did not start installation automatically

Sunny,

Open your Applications Folder and look for an Install OS X El Capitan application, if it located in the Applications folder, double-click on it.

Troubleshooting the Mac App Store - Apple Support may provide some clues.

Also, read can't download Lion OS - says it's been downloaded already, but it hasn't been for some additional options.

I downloaded El Capitan OS but it did not start installation automatically.

Technically it never does. It should open the installer, but you would have to press a continue button to proceed with the install.

The installer should be in your Applications folder. Double click it to start it up.

You are welcome...glad to help.