Late entry here but it might be useful to someone so here we go.
If your computer has been working with an external monitor but suddenly stopped supplying an image feed
Problem & Tinkering
I have a late 2016 MBP 15 running Catalina 10.15.4 connected ta an Eizo CG248 via a displayport - USB C connection. It has worked without issues for over a year. Then at one point I closed my mac and turned of the screen, opening the mac up again the screen is not recognized at all. Screen is blank and turns it self off after signaling "no input found".
As I used to work in a store selling apple computers I tried all the regular tricks. Rebooting everything connecting, disconnecting, trying to force find the screen. Resetting SMC and p-ram to no avail. I did have a hyperdrive, laying around with a HDMI out. Connected it and got an image but only with 30hz refreshrate that I wasn't able to change, so it wasn't a viable option, as I need 60. The hyperdeck specs said it would provide 60hz via usb out but for some reason it did not and the HDMI cord was a 2.0 one. Tried again with 2 different display to usb c cords. And bought a mini displayport to displayport. Through the hyperdeck also got a 30hz refresh rate. Other computers in the household connected to the screen fine. But not my work computer and it was really annoying.
I realized that there was one option I hadn't tried, if it was the hyperdeck that was resticting the HDMI i could try a HDMI - usb c cord capable of 60hz. So I got one and. I got an image.... At 30hz.
I thought I'd try again to connect the displayport - usbc and.... It worked. What?!
All the glory in resolution and refreshrate was back. And it has been working since!
So it seems that my computer for some reason had a hangup that was cured by connecting another type of cord, but in my case not through a hyper deck. I then returned the cables I bought since I didn't need them and now I'm back to status quo with my original hardware.
Go buy all the cord variations you can connect between your monitor and computer. Try them out one by one and retry your original connection in between. Then simply return the cords you don't need to the store again. If it works it works as It did for me. And it's free to try.
Hope you all get your image back!