Kdenlive was working so well (timeline won't play)

Kdenlive forum,

As I mentioned before, I was recently able to use Kdenlive to complete a full 10 minute video that had a fair amount of transitions and effects. It was a little bumpy, but I managed through it, and I was excited about using Kdenlive for my next video.

Unfortunately, it seems Kdenlive has different ideas.

When I create a new project, regardless of what settings I use or what kinds of clip I add, the time line video will not play. Nor will the clip preview. If I press play, it is frozen at the first frame. If I grab the timeline pointer and move it to somewhere in the middle of the clip, I just get a white screen.

The weird thing is that if I load up any of my pre-existing projects, they work. I can play, edit, and save them.

Any idea why it is the problem is manifest now, even though so far as I know I haven't upgraded or changed anything since my previous successful edits?