How do I turn on HomeSharing without original remote?
The Apple TV should be connected with a mini USB cable to your laptop USB. 
Go here: http://www.howtogeek.com/214243/how-to-factory-reset-or-reboot-an-apple-tv/ and scroll down to "If Your Apple TV Will Not Boot"

With laptop(and running iTunes) connected to Apple TV plug in the Apple TV to power. 

"If your Apple TV will not boot properly, or something has gone wrong and the unit isn’t operating properly, you can almost certainly restore it by connecting it to your PC and using iTunes to factory reset it.

First, unplug your Apple TV. You still need to power it back up, but before you do that, make sure to plug a mini USB cable in the back of your Apple TV (right below the HDMI port) and then to a USB port on your PC."

The link above even has pictures.