monitor enters power safe mode although there is HDMI input

I have contected an ST2010 monitor directly to a camera (video mode) via an HDMI cable input but without connecting it to a computer. The monitor goes through a cycle of showing the camera image than showing the sign 'Entering Power Safe Mode', switching off and then showing the image again. If I press the menu switch it says there is no signal from computer. Even if I set the input to HDMI the problem persists.  I have the same problem with another type of Dell monitor, however it has worked before with a BENQ monitor. Does anyone know a solution?

Thanks!