If the audio plays when you hit "play" then nothing has happened to it no matter what the pictures show.
Emptying the picture cache once in a while seems to help avoid the "flatline" problem in the first place.
Backing up projects to e.g. an external drive is always a very good idea just in case something does go wrong with the project, files get corrupted or accidentally deleted, the PC gets a virus infection etc. Losing loads of work because a disk has gone bad isn't fun either. So backup early and backup often.
The file saving versioning function and automatic saves are useful tools for helping to avoid disaster as well.