This problem is very intermittent/episodic, but when it happens, it can last for days before (seemingly) fixing itself. Basically, I wake my Thinkpad T420s (with Debian Jesse) and on random occasions, it won't detect the external monitor which is my main display. It's connected to the thinkpad's DP port via a HDMI-DP adapter. I've seen other posts on forums about xrandr not detecting external monitors on wake, but those solutions have never worked for me in the ~6 months I've been dealing with this (since I got this laptop). Other things which have not worked: re-plugging in monitor during sleep, then waking; rebooting; xrandr --auto.
Here's a few lines from Xorg.1.log that might start to explain things:
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[ 3888.623] (II) No input driver specified, ignoring this device.
[ 3888.623] (II) This device may have been added with another device file.
[ 3888.623] (II) config/udev: Adding input device HDA Intel PCH HDMI/DP,pcm=3 (/dev/input/event12)
[ 3888.623] (II) No input driver specified, ignoring this device.
[ 3888.623] (II) This device may have been added with another device file.
[ 3888.623] (II) config/udev: Adding input device HDA Intel PCH HDMI/DP,pcm=7 (/dev/input/event13)
[ 3888.623] (II) No input driver specified, ignoring this device.
[ 3888.623] (II) This device may have been added with another device file.
[ 3888.623] (II) config/udev: Adding input device HDA Intel PCH HDMI/DP,pcm=8 (/dev/input/event14)
[ 3888.623] (II) No input driver specified, ignoring this device.
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Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
LVDS1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
1366x768 60.00*+ 39.98
1360x768 59.80 59.96
1024x768 60.00
800x600 60.32 56.25
640x480 59.94
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
HDMI3 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
DP3 disconnected (normal left inverted right x axis y axis)