I looked up my dictionary and found "down" also refers to something doesn't work. For example, C.T. is down. Computer is down.
But can I say C.T. is up again if I mean the C.T. can work again? If no, how should I say correctly?
You can say "It's back up" to indicate that it had been down but now is working again.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Thank you very much for your replies. I got it.