I'm not sure if this is the best community to post in, but I just bought a used computer and slotted in an RX480 as the GPU. I installed KDE Neon 5.27 on it, and it worked flawlessly for 2 days.
Then, even though it was working earlier today, it slept and then would not wake up. So I turned off the power and turned it back on again, and was greeted with this error screen:
The only prior error message I'd gotten from the system was when I tried to install wine for one application, it told me some packages weren't up to date, without a way to fix it. I can enter the BIOS just fine.
You're welcome to use whatever init system you want, but Systemd solves a lot of the bullshit problems and limitations that come from init.d init scripts. Systemd also has a lot of its own bullshit and bloat, but it does an excellent job at actually being an init system and service manager if you know how to properly use it.
Sleep/wake issues with AMD gpu and platform drivers are super, super, super common. Fish back through your kernel journal after a reboot (journalctl -kb -1 should do it) and look for the driver errors immediately after the wake event. If this has been fixed in a later kernel release then update your kernel, if not go report it to either the Ubuntu folks or on the amdgpu gitlab.
It is mentioning gpu in the errors, so it would be the first thing I would try, to see if the errors change, because I have no idea what's going on here
Before doing anything, if your screen allows it, swap DP to HDMI or HDMI to DP as output, that may fix this to the point of being able to actually boot and further fix the issue.
I've had this before with drivers where suddenly it would fail on either port but would still run on one of the others.