I don't think there's any way to understand what's happening without quite a bit more information. First of all, please tell us the exact version number of PD18 you're working with.
It would also be very helpful to see a screenshot of your timeline before and after the issue occurred so we can see what you're seeing. If you can't recreate the problem, see if you can open up an autosaved version of the project from before the issue occurred. See this
post for more info on where the files are located.
Save each screenshot as a PNG or JPG then use the Attachments button below the forum's text box to share the images here: