I'm running Lubuntu 16.10 on a Lenovo ThinkPad T450s laptop. I have it connected with a VGA cable to an Acer V173 monitor. My machine dual-boots Lubuntu with Windows 10, and I can use the monitor fine in Windows, but Lubuntu doesn't seem to even detect it—it doesn't show up in xrandr. The same situation obtained when I tried an old Dell monitor. But, I had a different monitor working fine yesterday, before I flew from California to New Jersey (I unfortunately don't remember the brand or model). How can I debug this?
The problem has disappeared as mysteriously as it appeared. So, at least for the time being, it's solved.