HBO Now is no longer on my Apple TV what happened?

That is correct, HBO NOW was discontinued on the Apple TV 3's and earlier this month. HBO did notify customers however it seems you missed that.

If you want HBO NOW, it has been replaced by HBO MAX on May 27th. HBO Max is available at no additional charge on Apple TV HD & Apple TV 4K models. Therefore if you want to continue watching HBO content on an Apple TV you will need to upgrade.

It may still be working fine but that device has long since been discontinued and does not have the capability to run apps, only server-based 'channels'. Apple still supports it for a server version of their TV app and minor firmware updates, but many 3rd party providers have been abandoning their server counterparts in order to focus on the apps that run on current devices, HBO is just the latest one to do so.