This is not a problem but a sharing:
Yesterday I casually restarted my Thinkpad X380 just to boot to a black screen with a blinking cursor. Trying to get into TTY was a bit… “funny” pressing CTRL+ALT+Fx was showing a login screen for a split second and then returning to the black screen with the blinking cursor. No keyboard input was taken into this split second.
Fx*#! I was prepare to chroot and to attempt and update, or to replace some packages or to see if by mistake I have any nvidia drivers installed (google was pointing in that direction).
I do not know why I thought to check a ncdu and a gparted only to discover that my / is full… with… pacman cache…
needless to say that deleting the contents of the cache folder solved the problem. Now I only need to investigate why this was not automatically emptied… 
I hope this will help somebody, someday
p.s. my install is already circa 1,5 year old…