Before I start I should add a caveat - the post title is misleading. I didn't actually fix anything since there was no problem to begin with but my own lack of observational skills! Nevertheless there are many people similarly afflicted, and this post might help them.
So I’ve been connecting my MacBook Air mid-2012 model to an external ASUS VP228HE monitor successfully for a long time. That Mac doesn't have an HDMI port so I've been using a HDMI <-> Thunderbolt adapter all these years.
I just purchased a MacBook Pro early-2015 model, and tried connecting to that monitor the same way. However there was no signal to the monitor. So I was just getting a black monitor screen.
I scoured the internet for answers. I tried every bit of advice provided in this excellent Medium article, and several other articles, but nothing worked.
Eventually I realized that this MacBook Pro actually has an HDMI port! Duh… 😯
So I connected directly to that, rather than to the Thunderbolt port using the adapter. It worked beautifully.