Try following this support article and see if it resolves your issue, not sure why but this message can randomly occur sometimes and often this fixes it. Good Luck
This HDPC thind is a pain in the butt and so I'm going to post in most forums my fix.
Came home from work my computer was at login and when I logged in playing up and had difficulty signing in and restarting.
Now hang on a minute cant be my mnonitiorFound out later my son had blown a sandwich maker up a cheap peice of ***** my wife had bought and blown the circuit breakers on the house power so similar to blackout. Not that this caused the HDPC problem but led to it I am assuming.
So did all the forums etc etc did the rounds trying to find a fix to this sudden inability to play any movies TV shows or music through iTunes.
Now hang on a minute it cant be my Samsung monitor because Ive been wacthing these movies etc through it before ? Cant be the bluetooth speaker because alert sounds are coming through.
Can't be the TV because they've all worked before through ATV OK so found a piec on System Management Rest difficult with a Mac MIni and bluetooth keyboard and Trackpad but found the way to do that.
Didn't fix the problem but, I came across a piece in some small forum re iTunes being the culprit and it made some sense to me as I could still play this content through Quicktime, yes it wouyld all play on Quicktime.
So Deleted backed up my iTune Library , not hard it's on an external drive just ejected that then proceeded to delete everything iTunes from my cmachine everythjing I could find prefs etc in every library all out.
Did a complete clean install of iTunes latest vers mounted the exeternal drive with my iTunes media folder on it told iTunes where it's media folder was and hey bingo everything plays again no probs.
So Thats how I stopped the dreaded HDPC problem for me.
1. Check the stuff plays on Quicktime on your monitor if it does this then it's a big bet iTunes is the culpret.
2. If you suspect iTunes and not the monitor/s then do a complete delete after backing up content and a clean instal of iTunes
3. Restart your computer all should be good
A calssic symptom if everything was working OK before and no HDPC probs occured points to software as iTunes not your monitors or cables.