Cold water supply not flowing to mixer shower?

I have a mixer in my toilet and use it as a bidet, The hot water flows okay but no water from the cold. I noticed that the isolation valve was placed the wrong way up by a cowboy plumber and I thought this could be the problem. I have repaced the valve with a new one, the water is flowing but not coming through to the mixer only the hot again. The water come from my loft not the mains. Could this be the problem?