Target display mode on iMac stopped working after power outage

I have an old imac (late 2009 model, running High Sierra 10.13.4) that I’ve been using as a second monitor for a number of years now. But I had a power outage today and now I can’t get it working as a second monitor anymore for my iMac 5K, late 2014, running 10.15.6. Fixes I've attempted:

* resetting SMC reset on both machines

* restarted both machines in safe mode

* unplugging and replugging the display port cable

* ensuring my keyboard works and function key setting is proper

Still, command-f2 does nothing. I’m out of ideas. Anything else I can try? Is is possible the outage somehow damaged one of the machines? Is there anyway I might test for a malfunction?