Difficulty accessing subscribed channels on Apple TV

… but today I kept getting to an ad for the Charlie Brown halloween special.

That is a seasonal in-app promotion for specials that are included with the Apple TV subscription, but might be available for a short time to non-subscribers as well (like in previous years). A gift from Apple to Apple TV app users, if you will. ‘Thanksgiving’ and ‘Christmas’ are two other seasonal Charlie Brown specials that may be appearing soon.

Then I randomly started clicking and after giving up, the screen with all my channels shows up.

I assume you are referring to the Home Screen with all the various app tiles.

Now I'm afraid to select a channel because I don't know if I'll ever be able to get that screen back! Can someone please help an old lady with this? Thank you.

You can always navigate back to the Home Screen by using the Back (<) button [or ᴍᴇɴᴜ button] (depending on model) on the Apple TV remote, repeatedly [or use press-and-hold]. A short single press of this button would get you to a previous screen in the navigation hierarchy.

Alternatively, if you prefer, you may set a shortcut function to the TV button,

to either go to the Apple TV app (default), or go to the Home Screen (my preference). This is a preference that the user can set for themselves. Quickly go to the TV app or go the Home Screen - Apple Support

Settings﹥Remotes and Devices﹥TV Button﹥Home Screen | Apple TV app

(Note that long-pressing the TV button would open the Control Center overlay instead.)

Give this a look. Apple TV app at a glance - Apple Support It should help you get accustomed to navigating the App and the device. Just follow the links.