After being locked for a couple of hours my desktop stops responding and only a hard restart (hit power button) brings it back. In this state, it doesn't respond to ping, ssh attempts or keyboard/mouse input but fans are still running. In trying to wake with keyboard I have tried with both a logitech wireless keyboard + USB keyboard.

I run a cron job hourly so I can fairly accurately pin down when this happens. Looking at the logs from the last time this happened (I locked the computer ~4pm Friday, the final logs are from the cron job running at 2am that night, I then reboot at 11:37 Tuesday).

I'm running Fedora 27. There was no load on the computer between when I locked it and the last logs (which should rule out overheating). My power settings have automatic suspend disabled. I am running a Ryzen 1700X which I have heard might potentially be the issue?

Any suggestions are appreciated. If there is more info would help, please let me know.

I'm pretty sure the issue is discussed in this thread https://bugzilla.kernel.org/show_bug.cgi?id=196683. Issue summary is that Ryzen CPUs have issues with low power states. This is why the issues tended to happen overnight when nothing was running.

Simplest fix was to just permanently run something on one core, e.g.
stress-ng --cpu 1 --cpu-method queens
With that running, I have 6 days of uptime which is the best I have seen.

Better fixes suggested in that thread include disabling low power CPU states, increasing CPU voltage and/or updating bios and hoping something fixes it. But go read there for full details if you have the same issue.

It looks like every vendor will have issue bios updates.