There are some likely situations for this problem, but may be others
please see below but keep in mind that you may need to freh install and restore from backup.
The Issue seems to be that the rpi's bootloader needs to be updated. This is not something HAOS can do itself.
Grab a spare SD card and flash raspberry pi OS onto it and boot.
run the following commands
sudo apt update
sudo apt upgrade
sudo rpi-eeprom-update -a
this will update the bootloadeer and after this you should be able to run HAOS 15
The issue seems to be that the kernel version used in the update has a problem upstream. a potential fix has been mereged - home-assistant/operating-system#3957 infomation about the upstream problem - https://gitlab.freedesktop.org/drm/amd/-/issues/4050
This was a 15.0 issue and has been fixed in 15.1 so update to 15.1
A fresh install and a restore of most recent backup is likely the quickest way to get back up and running.