Hard drive's using the locking feature usually freeze up if they've been accessed without being unlocked, or have tried to be unlocked with bad keys. This is to stop you from brute-forcing the key (Eg, try key 1, 2, 3, 4, 5, 6, and so forth). After 3 bad keys the drive simply stops responding. I've heard before its even possible to permanently freeze a hard drive for maximum security but it isn't that frequent.
Usually a quick reboot will fix this. Sometimes you need to plug in the drive at the PC BIOS detection screen to bypass some other freeze causing commands.