Their system was set up such that when they rebooted the whole thing (which they needed to do to get out of the lockout Nerdy used, intending to steal the DNA samples, deliver them to his contact at the docks, then return without anyone realizing what had happened), it would first start up only using AUX power. Then they just needed to run a command to have the system switch to main power.

But they forgot because the whole island was a well-polished shit that they were barely holding together and hadn't ever trained on what to do after a reset.

After this scene, the power goes out through the whole park and to restore it, someone needs to go to the power station and manually activate the mechanism that closes the breaker to bring main power back on.

In the movie, IIRC they just skipped straight to the "start the power up manually in the power station", which Ellie does after Arnold fails to do so or return.

The book had a better system overall (where main power could have been turned on from the control room, or safely in the bunker if they had remembered it before the fences failed) and the issue was with a lack of experience with that system. The movie's version was simpler but a stupid system for a park full of dangerous predators because it didn't have a fail safe at all. Plus that stupid 3d interface that apparently Lex knew and was thus able to figure how to enable secondary systems when all of that would be custom software running on the OS.