All models on the h1seih23 computer stopped running.  A quick check showed that the I/O chassis appeared to be powered down, as no cards were reported from the I/O chassis with a lspci -v command.  We assumed the power supply had failed, so we removed the computer from the Dolphin network, stopped the models, and powered off the computer.  We then examined the I/O chassis only to find that it was powered up and appeared to be running OK.  

After examining the one-stop cable to make sure it was seated properly and showed no signs of having been kinked or otherwise damaged, the I/O chassis was powered down, then back up, and the computer restarted.  All models started normally with only a slight IRIG-B deviation into a negative value which quickly recovered.

Bottom line is we don't know what happened.