Files on external drive randomly deleted ? Yet still show up on df command

About one week before when I woke up I saw that most of the files from
my /mnt hard drive had disappeared. Public and personal archives, media
collection... Nothing shows up anymore. 'du -sh' command displays free
space, however the space is still used up as per 'df -h' command.

#pwd ; du -sh . ; df -h | grep -E 'Used|fusefs'
/mnt 122G .
Filesystem Size Used Avail Capacity Mounted on
fusefs 3.6T 3.3T 328G 92% /mnt #

Is there a way to diagnose the issue, and to keep/recover the
(apparently still present) files ?

--
Sylvain Saboua
www.saboua.xyz