What is the opposite of a server which is "down"?
When you say that "The server is down", you refer to a remote computer/program which is not responding.
How do you call the opposite state in which everything is working?
What is the correct verb to negate the "down" state of the server? for example: "Turn it back on", "Activate it" or something else which is more appropriate?
When a server is down, it is generally "brought back up" — the general form is "bring up".
I can't find a dictionary definition — it may be slang or jargon or too recent — but apart from my own experience working in computing, I did find a use online:
The next day, the astronauts rebooted the system, and ground teams reported that the computer was brought back up to full functionality with no further concerns.
— Denise Chow on space.com, 2011