Ask them to unplug and replug the HDMI cable, if this resolves the problem (often does) the what you have is a Hot Plug/EDID issue.
Usually this will happen when the display is turned off for several hours, many displays will turn off EDID management but not the Hot Plug return. There is a solution if the unplug/replug works. It is our HDM-AIO2 which will allow for Hot Plug control.