How is it possible to access a PC without a monitor with TeamViewer? I got a black windows when connected. with monitor attached I can access normally.
The graphics card doesn't output video when no display is connected therefore teamviewer can't capture and stream it. This is because without a display connected the graphics card doesn't know what the appropriate resolution is, and as a result outputs nothing/ a black screen. This behaviour seems to vary between graphics cards.
From teamviewer.com:
TeamViewer displays an exact copy of the graphics card's output. If the graphical output of the graphics card is disabled, TeamViewer may not be able to display the user interface of the remote device. Please make sure to connect a graphics display to the device. It may also be sufficient to connect a dummy plug to your graphics card.
In summary, plugging a dummy cable into the graphics card may trick it into displaying something, therefore allowing teamviewer to capture it, but success is going to vary.