You can't learn much by looking at its innards if it looks fine inside, without hard drive testing hardware.
The best way to find the cause of death is to plug it back in and see what noises it makes. Some examples are: clicking, grinding, no noise at all, which can all indicate different problems.
It also helps to diagnose by thinking about what happened immediately before it stopped working (such as a lightning strike), or if it was having problems for awhile or being slow for months before it finally gave out.
Some reasons hard drives die:
- power surges (e.g. lightning strike)
- mechanical failure (e.g. bad sectors, spindle broke, etc)
- dropped the hard drive
- too hot, exceeded hardware specifications and temperature limitations
- more rarely, too cold, froze and caused breakage or permanent corruption
- corrupted system (usually fully recoverable if only problem)
- defective PCB (motherboard) - common reason, often replacing PCB results in working drive again