Q: HBOnow app keeps shutting down on Apple TV

I am assumng you have an ATV4...

Force quit the app by pressing the home (tv icon) button twice then swipe up to force quit all the apps.

Relaunch the HBO app and try agan.

If the issue persists delete the app off the ATV by selecting then pressing and holding the touch pad until it jiggles.  Press play and select the option to delete it.

Reload the app from the app store.

Your only option in that case is a factory restore to eliminate the possibility of software corruption.

Restore your Apple TV to its factory settings - Apple Support

Did you Restore it?  A reset will not reload the system software.

If your ATV otherwise works normal, I would contact HBONow since they wrote the app and its their servers feeding the video.

http://help.hbonow.com/app/answers/detailHBO/a_id/116

Are you certain you did a Restore and not just a Reset? Reset just erases the settings. Restore actually downloads and rewrites the entire software image. This can take a while since the image is pretty large. If you have a computer with iTunes, it might be easier to download the file first and restore ATV this way Restore your Apple TV through iTunes - Apple Support

Since your built-in apps are misbehaving, it sounds like you need a full Restore. If you have already done a Restore, then please ignore these remarks.