I am using a brand new Lenovo P51 with 64-bit Windows 10 with a NVIDIA Quadro M1200 video card along with an external monitor (ASUS VS228H-P) connected through HDMI. When my computer goes to sleep, my second monitor turns off. When I wake up the laptop, but 2nd monitor is no longer recognized by the machine and I am unable to use the display. I have tried a number of solutions already:

1) I have tried selecting "extend display" under F7

2) I have tried using the detect display button in System/Display Settings

3) I have updated my drivers for both display adapaters (intel graphics 630, and NVIDIA Quadro M1200)

4) I have turned the monitor on and off,

5) I have cycled through the input options on the monitor while it is on

6) I have plugged and unplugged the HDMI cable

The only solution to reconnect the monitor I have found is to reboot the computer which is impractical for me while I am using the machine (I am running programs over multiple days and rebooting is not impossible). It is also impractical to turn off sleep mode on the machine - the whole point of having sleep mode is to save my screen and reduce power usage. I also use sleep mode for security - I want my laptop to lock so that someone can't just pick it up any time and use it. I also read somewhere that someone was able to turn off and then turn on the display adapters to reconnect - but this is a ridiculous fix I won't repeat over and over again.

Outside of these solutions, has anyone found a way to make Windows 10 recognize an external display?

I have also asked this question on the windows forum here: https://answers.microsoft.com/en-us/windows/forum/windows_10-hardware-winpc/2nd-monitor-will-not-wake-up-from-sleep/e09c8b69-de09-4314-8e56-11dde61117d9?tm=1501603727879

IN a similar situation. Yoga Pro-2 with Win 10 and external ASUS VS247H-P monitor connected via HDMI. I have to turn the monitor off and back on after Yoga goes to sleep. The Lenovo thinks its on but the monitor is NOT on.Did you find any solution?

Have you tried using a different cable, specifically miniDP->HDMI instead of a straight HDMI->HDMI cable?

The miniDP connector supports an HDMI monitor, but works differently than the HDMI connector.  Different handshake.

See if this makes any difference.

A different cable is an idea I haven't tried yet.

Since my original message, I've figured out that my monitor generally runs into handshake problems if I allow the computer to go to sleep with the monitor attached. My "fix" has been to turn off the 2nd montor if I am going ot walk away from the laptop.

In general I have concerns about the laptops video processor as well. It is hard to describe, but I have been getting weird patterns/salt pepper signals around my start menu which yesterday (for the first time) spread into the center of the display. A reboot fixed the problem - but it seems that sometimes there are problems with my display showing weird pixelations. I'm hoping its not my video card... I don't think it is a big enough of a problem yet to send in the laptop because I cannot consisently get the same error. I would expect Lenovo would look at it and decide nothing was wrong, and send it back. 

P51 with NVIDIA M1200 in a 230W dock.  2nd external monitor will not wake after sleep.   NVIDIA controller seems to think that the one working monitor is being controlled via both HDMI port and DP on the dock (looking in the NVIDIA control panel -> adjust resolution shows that the monitor is on the DP port, but the monitor only displays when selecting HDMI input).  Unplugging either the DP or HDMI cables from the dock disables the one working external monitor.  When I switch the cables around for the two monitors, on the back of the monitors themselves (not the dock), they both display again.  

Tried updating drivers for both video controllers (Intel and NVIDIA) as well as rolling back; disabling the Generic PnP monitor in Device Manager (no effect).  Disabling the NVIDIA controller does kill the one working monitor, but re-enabling only reactivates the one monitor.  Log (Event Viewer) shows an 'failed to migrate...' for the high def NVIDIA audio. 'Ambiguous definition' or such. 

So, at this point, it appears that the NVIDIA M1200 is not working with the P51 combatible dock - some kind of resource issue perhaps that does not refresh after wake.   Seemed to start after a windows update (Windows 10).    

Tried a different dock of the same model - no help.   No resolution as of yet....

Same issue, Workstation Dock, P51, Displayport Monitors, not displaying after sleeping only the monitor.  Discrete mode.

I too am having the exact same issue. I am running the same P51 wit the same specs as stated earlier. Has any found the true fix yet? 

Same issue here, P51 Quadro M2200, one monitor is plugged into the laptop via HDMI, second monitor plugged into the doc via HDMI, when the computer wakes from sleep the monitor that is plugged into the laptop is no longer recognized. Unplugging and reconnecting the failed monitor does not work. The only resolution is to reboot the laptop. Please help Lenovo.

I was having this same issue, and literally experimented as I found this thread and fixed it.  Fingers crossed it works for you.  My Yoga X1 is connected to the 135W Thunderbolt 3 dock, and monitors connected via HDMI were having the same "will not wake from sleep" issue.  I have two monitors connected and when I drove the built-in display (I usually keep it closed) at the same time it would wake just fine.  I found a setting in my monitors (an Asus and a D**l) for HDMI deep sleep.  Disabling this feature IN THE ON-MONITOR PREFERENCES fixed the issue and all HDMI monitors connected to the dock wake up as expected.