Try to identify where the most disk space is used by any of these commands:
Sorted list of disk space usage small to large:
# du -hs * | sort -h
Find files larger 5 MB (5000k, adapt to the size you need):
find ./ -type f -size +5000k -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'
Both of these commands may run a long while when executed from the root directory, because they will go through all of your disk.
It might be more efficient to start from /var/www/vhosts/ directory and /var/log directory, because most often a lot of space is occupied there.
You might also want to check if there are local backups stored on your system:
Find out where local backups are stored:
# grep DUMP_D /etc/psa/psa.conf
Then run du on that location, e.g. if your local dumps are stored in /home/dumps, run
# du -hs /home/dumps/* | sort -h
Adapt to your factual storage location.