YouTube The app is currently unreachable.

I am not familiar with that specific tablet, but if you have the Youtube app, and it just will not open, uninstall it, restart your device, then re-install the app. A lot of times updates, either to the app itself, the tablet software, or Android, can create slight incompatibilities in an app. A simple uninstall/re-install will often fix it. How to uninstall depends on your current software. On some of my Android devices I can press and hold the app icon which will open a little "App info" balloon. Tap it, and you will have an uninstall option. Otherwise you may need to go into Settings. If your Settings screen has tabs across the top, choose General. Look for something like Applications or App Info. Open that and scroll down to the app, tap it, and you will have an option to uninstall.

i encountered an error with Chrome browser after a regular update. Google Chrome was not loading pages correctly and giving the webpage error: ERR_QUIC_PROTOCOL_ERROR or saying "The app is currently unreachable" or This web page is not available" when using Google sites
The fix is quite simple:
https://www.errorsolutions.tech/error/err_quic_protocol_error/
Solution 1. Disable Experimental QUIC protocol

its reachable if you are having problem then reinstall the app.

You may need to go to the AppStore and download a different player such as : OPlayerHD Lite (Free). This 'should' fix the problem if you are not able to view flash enabled video. If the app you were using worked fine before, you could try reinstalling it. Sometimes that will take care of any issues.

Currently, only the Roku 3 supports Youtube. You can download the YouTube app through the Channel Store on Rokus that support it.

YouTubeThe app is currently unreachable.
what can i do

When linking your YouTube account to your Xbox 360, you should make sure that you are signed into your main YouTube account. When visiting the Activation page on your browser, the site will pull in the current signed-in YouTube account.

If you are not signed into a YouTube account, the site will prompt you for your email address and password associated with your YouTube account.

If you feel that the incorrect YouTube account is linked to the Xbox 360, please follow these steps:

1. Sign out of the YouTube app for Xbox 360.
2. In your computer's browser, sign out of YouTube and log back in with the email address associated with the account you want linked to the Xbox 360.
3. Complete the sign-in process on the YouTube app for Xbox 360.

Yep, good luck to anyone who can.
YouTube is currently not supported by the Zune HD.
If anybody is reading this who is considering buying an HD...Don't!
They have very low standards, they have a massive app store of 12-14 apps. they don't support YouTube, it's internet service stinks worse then decaying monkey brains.
Sorry to bash the HD.
To answer your question...the HD does not support YouTube and if your internet isn't working then it's most likely your WiFi connection, some WiFi Connections are not supported by the HD.
Sorry Mate
Hope it helps