Stop the container.
vzctl stop 101
Mount the containers ploop image.
ploop mount /vz/private/101/root.hdd/DiskDescriptor.xml
Find the partition for the ploop container.
fdisk -l /dev/ploop12345
Run the file system check and correct an errors.
fsck /dev/ploop12345p1
Unmount the ploop image.
ploop umount -d /dev/ploop12345
Start the container.
vzctl start 101
Sources:
https://kb.plesk.com/en/115683