No Audio

I am currently editing a video in Premier Elements 9 and whilst I was editing everything was going fine. I saved the project, waited a while, closed the window then shut my computer down. When I turned my computer on a few hours later and went back into the project I noticed that the audio track for the video was now no longer displaying the "squiggly lines" in the audio bar (audio bar was expanded to see these lines).

I checked that my audio on my computer was up. It was, 100% volume.

I checked the original video in my files and found that it had audio.

I have not muted the audio nor have I separated it from the video.

I also checked other projects and found that they did not suffer this audio loss. They all played the correct audio.

What can cause this to happen and is there a fix that I can do to get the audio to work again or do I have to start all over with a new project?