For some reason this happens regularly to me.
SETUP AND WHAT I HAVE DONE SO FAR.......
rpi 3 running latest and updated raspbian from usb connected Kingston ssd hdd (previously tried several brand new Sandisk uSD cards and same result).
Tried many different power cables and power supplies. Now using RS components 5V 6A with extra smoothing and decoupling caps added.
Tried multiple raspberry pi 3s with same result.
Static IP address is used via wired gigabit ethernet.
Also connected is a USB serial adapter connecting to an arduino pro mini that runs as a mysensors gateway for mycontroller system.
A large heatsink has been added with additional cooling fan which is temperature controlled via a small python script.
Generally it runs the cpu at about 5-10%.
It all works fine for many days or weeks and then just stops working.
It won't respond to pings, ssh or http. The activity led on the hdd no longer blinks indicating no read/write of the drive.
The 'activity' (green) led on the pi 3 flashes regularly about once a second and the pi network port green led flashes occasionally.
I have no monitor attached to this pi 3 to see what is happening locally. A power off/power on usually brings it back working again until the next failed state.
If anyone can help make this work reliably I would be soooo grateful.