With the update to kernel 6.18 RPi5 requires bootloader EEPROM 2025-02-12 or newer to be installed, otherwise LE won't work properly (eg you won't get any output on the screen).
Make sure your EEPROM is at this version, or newer, before updating to LE 13 builds after 2025-12-10:
If you are running LE12.2 or LE13 go to Settings->LibreELEC->Update, check Bootloader EEPROM update option at the end and install the update if needed.
Older LE versions don't ship with new enough bootloader EEPROM updates, if you want to update to LE13 either update to LE12.2 first or use a spare SD card and the Raspberry Pi Imager to create a bootloader recovery card and boot from that:
Download Raspberry Pi Imager from https://www.raspberrypi.com/software/ and install it, then choose device Raspberry Pi 5 and OS Misc Utilities->Bootloader->SD or NVME/USB or network boot (depending on your setup). Boot the RPi5 from this rescue SD card, after a few seconds you should get a green screen which means "success".