It's a multi-step process, and every system is different. It's not just a matter of flip the switch and wait for it to come up. If you're looking at something really primitive, you might even literally have to toggle in a boot loader program.

These 2 Youtube videos show how to initialize an S/390. The top comment on the second video is really good, too.

https://www.youtube.com/watch?v=ytMgyrZm87A

https://www.youtube.com/watch?v=qrKbh5HwF3Q