My guess is that you never watched the movie... COLOSSUS is self-powered with its own nuclear reactor inside on the mountain where it was built. The computer is deep enough - as is the reactor - that it is untouchable by humans.
Any technology sufficiently advanced would be indistinguishable from Magic."
Plus it had a Duracell 9volt battery for backup
shareVery early on, Forbin explains that the computer is self-sustaining and impenetrable.
It was designed to be super resilient and resistant to tinkering in order to protect it from sabotage by the enemy. In that regard, they built it TOO WELL. It protected itself from its countrymen, too.
And over time, the machine began implementing newer, additional security protocols in order to protect itself.
I do believe that this is the weakest link of the story: that they didn't include a kill switch or back door. I mean, EVERY computer at least needs a "reboot" sequence. They're notoriously prone to freezing up or going haywire.
But perhaps this film reflects a naïve time in our history when people still had blind faith in science and technology to achieve a state of perfection, even with computers?
They kinda-sorta felt that way with the Titanic, too...