Hard to say without knowing a lot more about your operating system, your processor, the source of your videos, the length of your individual videos, the length of your project, etc.
In other words, it's not something we can easily and consistently reproduce, so we'd need to troubleshoot your situation to find it why it's happening on your computer with your particular video.
This happens to me all the time. It's a memory allocation problem, and it's easy to fix. Basically, the more editing you do without 1) rendering and/or 2) saving as a video file, the more your computer has to keep track of. The following should solve the problem, and while a bit more time consuming, it'll make your job easier.
1. When your video starts disappearing, use Ctrl + Z to back up to the point where you can see all your audio/video.
2. Render your materials by hitting the Render button. (....be patient while rending takes place.)
3. Re-add the materials the you Ctrl + Z 'ed out. That should do the trick.
4. If rending doesn't work, go back to Step 1 (and get to the point where you can see all of your video again) and instead of clicking the Render button, click the Export & Share in the upper right corner.
5. Load the exported file, and finish your editing.
If it happens again, re-step the same process.
Ahhh, thank you, WillBeGoode! Thank you, Steve, for responding! These steps, WillBG, are easy to follow, and when I re-opened the program today all of the video appeared. Since I just learned that Adobe will not support me because I have APE 14, your responses mean a great deal to me. I am grateful.