Tried the USB-C connection again a week later after using a Display Port cable and it worked. Seems to have resolved itself.
Have A XPS 9520 that has been connected to a LG monitor. Has been working but has now stopped getting a signal. However, the laptop does recognise the monitor. Tried reinstalling driver, generic. System doesn't detect any other monitor if I try that. There are historical posts that this has long been an issue for Dell laptops and a usb-c connection. Tried same connection on a Dell monitor with same issue. Tried with different XPS 9550 and worked fine, same cable.  Does work as it should with Usb-c to HDMI. Any ideas on why this has started. Been a number of updates since but not sure which one may have caused the issue.
XPS 9520 external monitor usb-c no signal