[SOLVED] External monitor suddenly not detected

I rebooted my Slackware64-current laptop, and suddenly my external monitor stopped being detected. This was a normal reboot, not due to installing updates (so it was working previously on the exact same system configuration). The output of xrandr looks like this:

Code:dello@Latitude:~$ xrandr -q
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm
1920x1080 60.05*+ 60.01 59.97 59.96 59.93 48.04
1680x1050 59.95 59.88
1400x1050 59.98
1600x900 59.99 59.94 59.95 59.82
1280x1024 60.02
1400x900 59.96 59.88
1280x960 60.00
1440x810 60.00 59.97
1368x768 59.88 59.85
1280x800 59.99 59.97 59.81 59.91
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 56.25
840x525 60.01 59.88
864x486 59.92 59.57
700x525 59.98
800x450 59.95 59.82
640x512 60.02
700x450 59.96 59.88
640x480 60.00 59.94
720x405 59.51 58.99
684x384 59.88 59.85
640x400 59.88 59.98
640x360 59.86 59.83 59.84 59.32
512x384 60.00
512x288 60.00 59.92
480x270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320x180 59.84 59.32
VGA-1 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
VGA-1-1 disconnected (normal left inverted right x axis y axis)As you can see, the external monitor is not even detected. It also doesn't display anything during the boot sequence and prior to logging in, so I don't think it's an issue with the desktop environment (Plasma 5). To test whether the issue is with the monitor or cable, I tried with my wife's laptop also, which has identical hardware and is also running Slackware64-current but is a few weeks behind on updates. Here, the monitor works fine, and this is the xrandr output:

Code:irmamcfly@irmacpu:~$ xrandr -q
Screen 0: minimum 320 x 200, current 3360 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+1440+0 (normal left inverted right x axis y axis) 344mm x 193mm
1920x1080 60.05*+ 60.01 59.97 59.96 59.93 48.04
1680x1050 59.95 59.88
1400x1050 59.98
1600x900 59.99 59.94 59.95 59.82
1280x1024 60.02
1400x900 59.96 59.88
1280x960 60.00
1440x810 60.00 59.97
1368x768 59.88 59.85
1280x800 59.99 59.97 59.81 59.91
1280x720 60.00 59.99 59.86 59.74
1024x768 60.04 60.00
960x720 60.00
928x696 60.05
896x672 60.01
1024x576 59.95 59.96 59.90 59.82
960x600 59.93 60.00
960x540 59.96 59.99 59.63 59.82
800x600 60.00 60.32 56.25
840x525 60.01 59.88
864x486 59.92 59.57
700x525 59.98
800x450 59.95 59.82
640x512 60.02
700x450 59.96 59.88
640x480 60.00 59.94
720x405 59.51 58.99
684x384 59.88 59.85
640x400 59.88 59.98
640x360 59.86 59.83 59.84 59.32
512x384 60.00
512x288 60.00 59.92
480x270 59.63 59.82
400x300 60.32 56.34
432x243 59.92 59.57
320x240 60.05
360x202 59.51 59.13
320x180 59.84 59.32
VGA-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 connected 1440x900+0+0 (normal left inverted right x axis y axis) 408mm x 255mm
1440x900 59.89*+ 74.98
1280x1024 76.00 75.02 72.05 60.02
1280x960 60.00
1366x768 59.79
1360x768 60.02
1280x800 74.93 59.91
1152x864 75.00
1280x768 74.89 59.99
1280x720 60.00
1024x768 75.03 70.07 60.00
1024x576 59.97
832x624 74.55
800x600 72.19 75.00 60.32 56.25
848x480 60.00
640x480 75.00 72.81 59.94
720x400 70.08
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-2 disconnected (normal left inverted right x axis y axis)
DP-2 disconnected (normal left inverted right x axis y axis)
HDMI-3 disconnected (normal left inverted right x axis y axis)
VGA-1-1 disconnected (normal left inverted right x axis y axis)The laptops run Intel graphics but also have a dedicated AMD card that is only used when specifically activated. Since the issue first appeared, I did install the latest updates (which included a kernel update) and re-ran lilo, but that didn't help. Any ideas what the cause could be for the monitor not being detected or how to fix it?