It happend to me. I wanted to install a package, clicked yes once too often and it wanted to download the full Gnome desktop together with an outstanding update. I canceled the installation with Ctrl+C.
Of course, a reboot doesn’t work because the kernel is updated but the initram wasn’t built yet.
Only thing to do: use a LiveUSB image, use manjaro-chroot and to the update manually. Most of the time, pacman still works, but even then, I fixed it with a statically compiled version.
That’s one of the reasons, I always want a log visible on screen when doing an update, so that I can see what takes forever. (Maybe it’s installing nerd-fonts-complete which is over 2GB to download and a lot more to build, package, and finally install.)
On the other hand, I understand that using the pamac GUI is more convenient and “normal” users don’t want (or need) to see the specific update log.
The journal usually has the log, there should be something why it happened on your system.
Also,
Also, never to that. Instead to this: [HowTo] reboot / turn off your frozen computer: REISUB/REISUO