Use a live iso and open a terminal
sudo manjaro-chroot -a
Then check if you have these files
# /etc/shadow-
# /etc/passwd-
These are backups - list their content - they should contain your username
# cat /etc/passwd-
# cat /etc/shadow-
If they contain your username copy them to their original
# cp /etc/shadow- /etc/shadow
# cp /etc/passwd- /etc/passwd
Edit: 2020-09-01T05:40:00Z
Another candidate for verification is
- /etc/group