Are you certain the problem is with the kernel update and not some other application updating ?
Did you try to manually display/drive the external monitor using variations of the xrandr command?
To do so, try something like this:
(1) with external monitor NOT plugged in, type as a regular user:
xrandr
and make name of what your monitor is called, and make note of what resolution/frequency your monitor is using. For example, lets say you get LVDS (as your laptop monitor name) and that it is using 1024x768 at 60 Hz . (your resolution will likely be higher, this is just an example).
(2) Then plug in your external monitor.
Then again type (as a regular user):
xrandr
and make note of what name your external monitor is called, and look for a resolution that is the same as what you had with no external monitor. Lets say you also see it called “VGA-0” and you also note a 1024x768 at 60 Hz resolution.
(3) then to clone your display as a regular user type:
xrandr --output LVDS --auto --output VGA-0 --auto --same-as LVDS
or if you wanted to display to the right you could type:
xrandr --output VGA-0 --auto --right-of LVDS
(4) and to stop the external monitor you could type (as a regular user):
xrandr --output VGA-0 --off
= = = = = = =
There is a guide here: Xorg RandR 1.2 - ThinkWiki and to get more help :
xrandr --help
or even better:
man xrandr
where if you scroll far enough in the ‘man’ page you will a number of documented examples.
If xrandr works then it is likely not the kernel but rather some other update that broke this.