Having this kind of crash between to boot without touching anything is very weird...
Anyway, i will explain you to restart from the start to be able to boot properly.
First to know your version of any program on your system
# pacman -Qs s6-boot
this will display a little information about s6-boot like this :
local/s6-boot 0.2.1-3 (s6-suite base)
Necessary files to boot under S6 supervision suite
do the same for s6opts. This is a basic command of pacman, please see this page for more information about pacman :
https://wiki.archlinux.org/index.php/Pacman
You didn't answer me about your current symlink...
Well, normaly you have a database called Default at /etc/s6-serv/enabled/rc/compiled. Redirect the current symlink to this one :
(following command is made with root permissions)
# ln -sfT /etc/s6-serv/enabled/rc/compiled/Default /etc/s6-serv/enabled/rc/compiled/current
Doing this command should able you to boot properly, but be aware that any kind of service will not be started at the boot time like dbus syslogds6.
So, reboot.
Now, you need to rebuild properly your database used previously. I don't know which database you used, i will assume for the example that it's Live.
First to know which service are on it :
# s6opts list Live
this will return something like
current -> Live
previous -> Live.backup
Rc longruns service(s)
alsa-log
alsa-longrun
connmand-log
connmand-longrun
connman-vpnd-log
connman-vpnd-longrun
s6rc-fdholder
s6rc-oneshot-runner
wpa-supplicant-log
wpa-supplicant-longrun
Rc oneshots service(s)
alsa-oneshot
Rc bundles
All
bundle-Alsa
bundle-Connman
bundle-Wpa-supplicant
obviously this will depends of your database.
To know the source used to compile this database use :
# sudo s6opts which_source Live
this will return something like
:: Source of Live database
/etc/s6-serv/enabled/rc/source/live
Now destroy this database and the source of this database
# s6opts remove_db Live
this will remove the compiled database
# s6opts remove live
this will remove the source of the database
Now, rebuild it properly
(adapt this example to suit your needs)
# s6opts add live bundle-Alsa bundle-Connman bundle-Wpa-supplicant
this will create a new source of database called live with bundle-{Alsa,Connman,Wpa-supplicant} on it.
and finally build a new database called Live with this new source
# s6opts compile Live live
Now, you can switch on it to be able to boot with it
# s6opts switch Live
You can try to reboot.
Apparently , you have an another strange error with your user database
s6-svc: fatal: unable to control /run/boot/service/mdc: s6-supervise not running
if the trouble persist after doing the command above, please post your /var/log/user-supervision.log.