I have a Dell Optiplex 780 DT with Windows 7. This setup worked perfectly with 2 monitors but one day it crashed. I installed a new Windows 10 Pro. Now the 2nd monitor is not recognizable and the video controller in Device Manager has a yellow triangle symbol, warning that it has a problem. Driver update doesn't work; it generates a "There are no compatible drivers for your device." error message. How can I fix this? Do I have to buy a new video card? I searched for 1-slot video cards that support two monitors in Windows 10 and was not successful to find any.
Dell's drivers page says that the drivers are for Windows 7, and they won't be releasing new drivers for Windows 10.
Nevertheless, you can download the Windows 7 driver and try to install it on Windows 10.
It might not install automatically, but Dell says that a manual download and install could work.
If the driver really doesn't work, then you need a new graphics card which has Windows 10 drivers, or you need to go back to Windows 7.
You may have been using the wrong search terms in your graphics card search. Basically any modern NVIDIA or AMD Radeon graphics card will let you use two monitors.