Sharing this in case it ever helps anyone else ...
My main PC is a Thinkpad laptop which sits on my desk, connected via USB-C to a dock which provides power, link to external monitor, additional USB3 ports, etc.
This morning, switched laptop on, and external display didn't wake up. Odd.
- Maybe it's switched itself to another input? Checked, reset, and confirmed it's on the right input but not receiving a signal from the dock.
- Connect laptop direct to monitor, and all is OK, so cable is undamaged and laptop is outputting a signal.
- Unplug, switch dock off and leave for a few mins, reconnect the peripherals and switch back on. Still no monitor signal.
- Hmmmm.
- Then I notice laptop battery level is falling and is below the level that should trigger it to charge. Hmmmm some more.
- Check battery charge settings, and it should certainly have started charging as it's now <50%.
- Try a different USB-C cable from dock to laptop, still no change.
- Run Lenovo diagnostics and there are a few driver and firmware updates recommended.
- Install, reboot, and battery is still not charging.
- Hmmm some more.
- Google for local laptop repair options.
- Let's make sure all docs/settings are backed up to date.
- Then let's remove any sensitive docs completely.
- That hits the battery level, now <30% after some intensive backup activity.
- Try the thunderbolt port instead, still via the dock, and still no battery charging.
- Dig out the separate AC adapter, plug that in, and still no battery charging.
- One last Google search reveals an "emergency battery reset button".
- Switch laptop off, insert pin into (well hidden) reset button recess, hold for 7secs, release and switch back on.
- Hold breath whilst laptop boots.
- Login, still holding breath.
- Check battery status, and it's charging again.
- Working both through dock and adapter.
- Switch off, leave it charging, and walk away for an hour or so.
- Switch on, and battery is restored.
So, if you ever have an issue with charging/powering a laptop, check for the presence of an emergency reset button. Or check now, so you know in advance.