So I have a late 2012 Mac Mini, I have had the same issue using Mavericks, Yosemite and El Capitan. If my Mac Mini goes to sleep for long periods of time (over an hour for example) my monitor will not power on when the system does. I know the Mac wakes as I can remote desktop into it, which also turns the monitor on. But when I disconnect from remote desktop the monitor goes off again. So for the last 3 versions of OS X I have had to disable sleep due to the monitor not coming back on. I always do a fresh install of OS X between versions too. Anyone else experiencing this? or had something similar? Anyone know of any possible fixes?