How can I preview all of my video strips? I have pressed something, not sure what... and now I cannot preview all of the strips in my video, eg; I can preview sound, but not image. If I add a new clip, then this previews fine. 

The strips I can't preview seem to be faded out in the video sequence editor, how do I get it back? see image!

Thank you in advance for your help :)
Just select the faded strips and press Alt+H and the hidden strips i.e your transparent strips, will be unhidden. You probably pressed H which is used to hide.