Q: External Monitor Won't Wake Up from Sleep

Unplug any non-Apple USB devices.  Many don't understand Apple's Energy Saver.

Is the imac that is in target display mode also sleeping?

FWIW, some related articles that talk about sleep in target display mode:

Using a 27-inch iMac (Late 2009 or Mid 2010) as an external display

Target Display mode may not work after waking from sleep

I have the new 15" Retina MacBook Pro connected to a Thunderbolt display using one of the TB ports. Earlier today, I discovered that I could connect a second older 24" Dell Monitor (2405FPW) I've been using for years by using the DisplayPort to DVI adapter I had on my old iMac, and plugging the Dell directly into the second Thunderbolt port on the MBP.  All was well, until the displays went to sleep (I have my laptop configured to not sleep for 3 hours, but have the displays sleep after about 15 minutes).  When I came back, moving the mouse and typing on the keyboard brought the Thunderbolt Display back to life, but the Dell monitor remained dark.  Pressing buttons on the Dell had the message "In Power Save Mode - press key on the keyboard" appear.  I tried turning the monitor off, back on, unplugging the DVI cable from the MBP, all sorts of things.  It just would not wake up.  When going to the System Preferences, the dell WAS recognized.  As I plugged and unplugged the DVI cable from the second TB port, the "Arrangement" tab would appear and go away.  So, it SEEMED like it was working.

Final solution for me: I selected "Sleep" from the Apple menu to actually put the entire machine to sleep.  I then jiggled the mouse, and the machine woke up, along with BOTH monitors!

So, it appears that if just the monitors go to sleep, the "wake" signal is not making it to the Dell.  If the ENTIRE MACHINE sleeps, they all wake together.

Hope that helps someone.  I couldn't find an answer, so I found it trial-and-error.

Wow - I had the same problem here and tried EVERYTHING.  I finally unplugged the video and power cable for TWENTY minutes and boom - problem solved.

I had been using the Dell monitor fine for the last 6 years until this.  Crazy.

When you put your Mac to sleep, wait for the external monitor to show "no signal" or go black. Then switch the monitor off.

Then, before you wake up your Mac, switch the monitor back on, then wake up your Mac. This is clearly some sort of Apple Thunderbolt/DisplayPort problem.

There are at least 20 threads in here with similar Thunderbolt/DisplayPort problems.

I have the exact same problem.  Only solution I found was to unplug/replug or have a sleep corner and re-wake everything.  Man what a major pain.

Apple why haven't you guys fixed?  It's been around since mid OSX 10.8

It has been reported over and over and over.

However, there are 1,000s of external monitors, each with different wake up tendencies.

What I hate is the main monitor flashing and moving 2nd monitor windows back to main monitor

Feck

Stupid thing was it was working fine, then an OSX update way back in Jan of last year the monitor stopped waking up.  So either something broke in the monitor or the signals/timings the laptop is sending got changed.

The "A" team at Apple now works on iOS. The "B" team are stumbling around, it seems to me.

I'm betting it's on the to-do-list and they've got 1,200 other things at higher priority.

Like fixing Numbers 3 so that the X-axis can be set to a date!  *nudge nudge wink wink*

I do own an Apple CinemaDisplay 23" Alu connected to mini displayport with a DVI / mini displayport adapter, on a 15" retina mbp 2013, and have the exact same problem.

What is weird is that I have another mbp, 13" 2011, with the exact same setup, and the problem never occured.

So it may be linked to retina actually ?

What's sad (and maybe a shame), is that Apple is not abble to correct this problem with its own hardware setup...