1. YouTube channels are closed when their users close them via the Settings / Overview / Advanced page at www.youtube.com which results in this:
2. they are closed where the user closes its related Google account:
3. Of course there is a chance that removing a new Google account's Google+ profile did this but unless you can backtrack and recount what you did to arrive at this point it is all guesswork on my part. ;)
1. is an unrecoverable-from scenario- YouTube channels which are user-closed are perma-closed,
2. may be doable if you were to have the Google account reopened. You will need to use "can't access your account?" at www.google.com,
3. may be recoverable from if you were to reinstate/activate the Google+ profile. For more help with that check their forum.