If a space ship has been inactive and un powered for a long time, maybe days, weeks, months, or years, there might be a complicated procedure to start it up.
I once read that large solid fuel rocket motors were tested in stands by igniting a smaller rocket inside them. The exhaust from the smaller rocket ignited the solid fuel and solid oxidizer and started the larger rocket burning. So how did the smaller rocket get started?
If someone lit a match to light a solid fuel rocket dangling inside the nozzle of a giant solid fuel rocket, he wold be inside the giant rocket's nozzle and would be burned to death if the giant solid fuel rocket ignited before he could get out and far enough away in time.
So possibly a another, even tinier rocket would be used to ignite the second rocket which would be used to ignite the giant solid fuel rocket being tested. But eventually there would be a first rocket, which might be ignited by lighting a long fuse from someplace far enough away to be safe, or by remotely activating a device that made a spark, or by remotely opening a valve causing two substances to mix and spontaneously ignite, or remotely activating a liquid fuel pump, etc.
And starting a space ship's main power generator might involve a similar sequence of less energetic actions causing more energetic actions which cause even more energetic actions and so on.
Presumably a fusion generator would need a lot of electricity to operate.
Maybe some chemical fuel might be burned in oxygen to run a little generator that provides energy to run a fission reactor. Electrically powered machines would move the fission fuel and the control rods, etc. into place and pump the coolant, and presumably steam produced by the reactor would turn a generator which generated many times the electricity that the chemical powered generator did, which could now be turned off as some of the fission produced electricity would be used to run the fission reactor.
so most of the electricity from the fission reactor and generator would be used to start and run a small fusion power plant what would generate a lot more electricity. Some of that electricity would be used to run the small fusion power plant, so the fission reactor and generator could be slowly closed down until needed again.
Most of the electricity from the small fusion power plant would be used to start and run a much larger fusion power plant. Some of the larger fusion power plant's electricity could be used to run itself, so the smaller fusion reactor could be shut down until needed again.
And repeat in as many step as as are necessary until the ship's main fusion power plants are up and running and providing all the tremendous amounts of energy needed to run the life support systems, and the weapons systems, and the anti gravity, and the slower than light space drive, and the faster than light space drive, etc., etc..
Note that fission fuel would gradually decay to non fissionable elements with time, so a fission reactor would become useless for a step in this start up process after a period of time which depends on various factors that atomic energy experts could discuss.
At a spaceport, the energy for the first step in the start up procedure might be provided by the spaceport's power generators. An exploration or war ship would not expect that service to always be available when needed and so would be equipped to start up entirely on its own.
Of course if some adventurer lands at a space port and suspects that he might possibly have to run for his life back to the space ship and take off just a few minutes after landing if things go wrong, the space ship wouldn't be turned off and un powered but left with the engines idling, more or less, so that it might be able to take off and escape almost instantly.