Another is a corrupted filesystem.
But for that, you'd have to remove the SD card and insert into another Linux client that has the EXT4 filesystem stack available.
You would do a:
dehakkelaar@laptop:~$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 111.8G 0 disk
├─sda1 8:1 0 190M 0 part /boot
├─sda2 8:2 0 37.3G 0 part /
├─sda3 8:3 0 69.9G 0 part
│ ├─vg0-lamp.dehakkelaar.nl--swap 254:0 0 1.5G 0 lvm [SWAP]
│ └─vg0-lamp.dehakkelaar.nl--disk 254:1 0 10G 0 lvm
└─sda4 8:4 0 4.5G 0 part [SWAP]
sdb 8:16 1 7.2G 0 disk
├─sdb1 8:17 1 256M 0 part
└─sdb2 8:18 1 7G 0 part
sr0 11:0 1 1024M 0 rom
And run filesystem-checker on the SD card partitions:
sudo fsck /dev/sdb1
sudo fsck /dev/sdb2