HDMI Port not working after update

Hi

I have a Mac mini that I am running three screens from. It has all worked brilliantly until I did the update to El Capitan last night. Now the Screen running through the HDMI port does not work. The other screens are plugged in through a USB Adaptor and the lightning Port. Like I said it all worked fine until the update- any ideas? The HDMI screen does not appear anywhere!