How to restore an ext4 filesystem
First you can test your hard drive via the package smartmontools
Activate "smart" in your drive :
smartctl -s on /dev/sdc
Start a long test ( you can try in first "short" test ) :
smartctl -t long /dev/sdc
Wait end of test and get result :
smartctl -l selftest /dev/sdc
If your hard drive is ok, you can try command bellow to check filesystem :
e2fsck -n /dev/sdc1
If it doesn't work, testdisk is the solution.