Do you have a working X (xorg)? On the archwiki for kde they advise: "Before installing Plasma, make sure you have a working Xorg installation on your system"
You might try hitting a function key (F1 - F8). If you get a command-line prompt, log in as root. Using pacman remove Plasma
and then (following the installation wiki) make sure xorg is functioning.
I personally find xfce4 a nice desktop. KDE is quite bloated. At first you might have to use the "startxfce4" command to get X to work. But you can easily make it automatic after log-on (again, following the arch wiki).
Here is the webpage for arch forums:
https://bbs.archlinux.org/ They obviously would be a great resource for Arch problems.
I have been using Arch for 3 -4 years. I found that the beginner's guide to Arch Linux installation was the best way to get Arch up and running. By the way, it is a great system and with rolling-upgrades you are always on the bleeding edge (minus the blood). Most Arch users are quite willing to help but many of them insist that the user do his due-diligence before asking for help (which sometimes is bothersome). Here is the url for the Beginner's guide:
https://wiki.archlinux.org/index.php/Beginners'_guide
Good luck -- and hang in there. It is an excellent system -- and you will learn a lot as you put your own system together. It is definitely worth the effort!
