If you have an unresponsive Nexus 7 touchscreen due to a crack and you can still see all the images displayed, you can still use the device until you can send it in for repairs. How? Well, you will need access to an OTG Cable and a USB Mouse.
Before you send in your Asus Nexus 7 (2012) for repairs, you will want to wipe your data since the entire unit could be replaced instead of being repaired and sent back to you to minimize repair time.