Dell XPS15z HDMI connection problems - help needed please.

I have a xps15z and the hdmi connection suddenly stopped working about a week ago. I have tried everything I can think of to get it working again, reinstalled drivers, tried different leads, different monitors but no success yet. 

The weird thing is when you plug in the hdmi lead the external monitor is detected on the nvidia control panel and the windows display settings. Also I can scroll across to where the extended screen is and drag windows over there but nothing shows up on the monitor. The monitor says there is no connection. 

I have been tearing my hair out trying to fix this for the last few days so any help would be really appreciated.

Thanks in advance!

relevant system specs: windows 7, nvidia Geoforce GT 525m graphics card (runs the external display), Intel(R) HD Graphics family (runs main laptop display).