when compositing 2 videos with greenscren I dont get a render result, other than blac

Hello all,
I have a problem that sounds weird- I just made a greenscreen videos in composite mode, - it works great,I have output nodes set to view and composite, but I do not get any output. In default scene there is only the default scene , - i.e. totally black when rendered, but outside of composite view there is no output, when I render in cycles I get a totally black movie.
I have looked into all menus but no chance, I cannot render the output although the output is ok, just what I wanted.
Can anybody help and show me which gross error I am doing?
P.S. I use blender 2.77 on Ubuntu 14.04

Is Use Compositor checkbox enabled in Render > Post Processing panel and is the result of composite connected to input of Compositing node? Also delete the RenderLayer node if you don’t have anything in 3d scene that must be rendered.

Hello, no sorry, - I have checked composite node input correctly, and what is strangest for me is - in composite mode there is a window for UV, and in this window there is the rendered image to see, -sometimes, sometimes it is also blank.
I have no idea which checkbox i have wrongely checked, -but maybe I cannot see the error because I have looked to long.

Hello - problem solved - in postprocessing there was not only composite box checked, but also sequencer - when I unchecked sequencer it worked perfectly - no problem at all!