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.