How do I reinstall Lineage OS or Android?

I installed LineageOS on Android once, but when I tried to lock the bootloader, the OS disappeared and I couldn’t boot. Then, when it starts up, a black screen “https://openattitude.com/wp-content/uploads/2015/10/wait-01-6p1.jpg” This image is displayed, from which the circle circle of Lineageos I can not start just round the wheel. How can I use Lineageos?
My iphone is Nexus5x.

AFAIK most (almost all?) devices don’t allow you to lock the bootloader with a custom ROM (i.e. LineageOS). If you want to use a custom ROM, you have to keep the bootloader unlocked.

You probably need to unlock the bootloader again and then reflash LOS.

How do i do? By the way, I can not use Android. Everything can only be done with a PC.

depends on your device i guess, look in your devices sub forum on xda, im sure you’ll get your answers about bricked devices, unlocking bootloader and reflash whatever you want

My iphone is Nexus5x.

Definitely check the xda forums for N5X. You should be able to relock the bootloader (and do a factory restore) via fastboot.