Mac Pro won't sleep any more

The past couple days, command-option-eject no longer puts my Mac Pro to sleep. I haven't done any software updates in that time. No process seems to be doing anything very interesting, or CPU-intensive.

What's stopping my Mac from being able to sleep? How can I figure out what it is?

Open Terminal application in your utility and type

syslog |grep -i "Wake reason"

You shortcuts keys must have changed (with or without your intervention)

Make sure you have no hot corners enabled to keep it awake (System preferences Desktop > hot corners)

Using the Terminal application on macOS Sierra 10.12 or later you can run the following:

log show --last 2d|grep "Wake reason"

This will provide a list of the various reasons your system is not sleeping. Unfortunately the list will likely contain a few things that are not immediately clear, but a quick search on those messages should lead you in the right direction.