I was editing together some clips in the VSE. All was apparently well, but coming back it this evening, I find that only two strips are audible and only a few visible. I can't think what I might have done by accident to cause such a thing. If it helps, the audible strips are soundtrack strips which I added last. No apparent rhyme or reason to the invisible video strips. I am running Blender 2.77 on Linux.
Help would be much appreciated if you have any thoughts. Can you think what I might check to fix the problem?