Well, as you did not precise which Desktop Environment you use, I can tell what I did a few days ago in a similar situation.
I reinstalled a fresh Manjaro Gnome version last weekend, and wanted to get rid of Plymouth. I disabled all systemd services related, and uninstalled all Plymouth packages. But as some of these are dependecies of critical packages, I ended with broken GUI system at reboot, with TTY working.
I got a look to Pamac history, took a note of all the packages removed (gdm by ex), reinstalled them and finally enabled gdm service.
Well, I’m on xfce4, so I don’t have these silly issues!
Ah yeah I haven’t changed anything system-wise in a while, so likely what Aris pointed to. Thanks for the pointing this out, who knows!
And look, I see this thread as a positive contribution, I’m sure another dummy somewhere will panic with out UI upon restart at crunch time and will start googling thing, perhaps this thread lesssens the chances the aforementioned dummy(ies) will create yet more threads and instead be redirected where they should be from here!
My first tries at Linux, even with Manjaro, when I had panic attacks after a (seemingly) broken system, I ended up reinstall all from scratch.
After a while, when you know more and more about your system (and whith help of this great forum too), there will be less and less times when you have to give up and reinstall your system. You’ll always (likely) find a fix.