Secondary Monitor Suddenly Stopped Outputting - Has Worked Previously

Hi,

Whilst playing a browser game on my primary monitor (Samsung SyncMaster S24B300), and watching a YouTube video on my second monitor (Samsung S22D300), the second monitor (S22D300) suddenly stopped displaying output. The monitor is still powered on (still has a blue light etc.), and it's also NOT on stand-by, but it doesn't display anything.

Interestingly, whenever I go into screen resolution and switch between primary monitors, the right monitor which isn't displaying output, does show the HDMI logo at the top left of the screen, which shows that it's acknowledging changes I make.

The monitors have been working side by side for several months, and this is the first issue I've had with it.
The primary monitor is connected via HDMI directly to the graphics card (NVidia Geforce GTX 660TI 2GB), and the secondary monitor is connected via HDMI to the motherboard (ASRock Fata1ity Pro). I understand ideally they'd both be going to the GPU, but my GPU only has one HDMI port and it's been working for several months without issue up until now.

All specs are up-to-date.

Things I've tried:
All cables are securely attached.
I've rebooted.
I've rebooted the monitors themselves.
In Device Manager, both monitors are recognised.
In Device Manager, I've disabled and re-enabled the monitors.
I've checked the settings on the monitor in question which works as expected; again I can change source to HDMI but I only get a lit up black screen - no actual display.

Edit: Please also note that I can still drag windows over to the second monitor, I just can't see anything displayed.

Any and all help much appreciated.

Cheers,

your Typical Nerd.