Second Display stopped working.

Hi all.

So I have been using my PC all day on and off. I just woke my PC from
sleep half an hour ago and the second display wouldn't come on. It does
not show up in Settings > display Settings any more.

The gpu is a Nvidia GTX 760 graphics card with 2x Dell 24 inch monitors.
The first is connected by HDMI, the second by Display port.

I rebooted. Still not working

I ran sudo apt upgrade then rebooted. Nope.

I went into additional drivers and removed then re-installed the Nvidia
driver. Nope.

Still not working.

If I go into Nvidia settings the monitor on displayport shows up but
will not let me enable it.

If I reboot to windows 7 everything works as before so it mustn't be a
hardware problem.

What on earth has happened?

I was using both monitors. Put PC to sleep. Wokle it up and bam! I am
stuck with this!.

What can I check to diagnose this?