folders in NTFS drive suddenly disappear

On a Dell PowerEdge 2300 server running Win NT4 SP4 with two drive partitions (C: FAT, E: NTFS), many folders within the E: drive, amounting 5 GB suddenly disappeared as if they were moved / removed. Permissions were correctly set and there’s no log of someone deleting/moving the folders. A thorough AV scan was done and no virus was found. Login in with Admin rights did not render different results, in case the files were hidden. Drive propierties displayed the freed space. We had to restore from backup. I am leaning towards a power-related question. Any other ideas?