This means that your video or other media clips have been moved or the connection to them has been disconnected.
Remember that, when you import video into your project, you are actually just importing a link to your video -- so your video and other media clips must be on your computer and available whenever you are editing.
A common mistake is to connection video that is still on your camcorder -- and when you disconnect the camcorder you lose your connection to the video. This can also happen if your video is on an external drive.
Where are the media files that you are using in your project located on your computer?
I'm aware of this as I have made that mistake before, however I know I haven't moved or deleted any of the files I'm using in my project off of my computer or off my memory stick as I've been working on it on and off for the last week and it's all still been available.
If I have to start my project again from scratch, how can I know/ensure that this won't happen again and my work will be saved?
I can't say without knowing where your media files are being stored.
SOMETHING is clearly changing and breaking this connection. This can happen when you've got a memory stick or external drive that hasn't been mapped and assigned a drive letter or if you've briefly disconnected the drive and Windows doesn't recognize it as the same drive.
So you'll need to do some detective work.
Is every file in your project disconnecting or only some files? What do those files have in common?
All the media files are stored both on my memory stick and my computer, and they're also in Adobe Organiser too.
Some of the files in my project are still there, some say "media offline" etc. and some are missing altogether. I think the ones that are there and working still were all taken and imported to my computer on a different day to the ones that are missing/offline.
You shouldn't have these files on a memory stick. What happens if you move all of your files onto your computer's drive?
And can you PLEASE tell me in what directory on your hard drive your missing video is being stored? Just saying you have media files stored on your computer doesn't give me much to work with.
Sorry, I'm not sure what you mean by "directory on my computer's drive?"
A directory is a folder on your hard drive.
For instance C:/Program Files/Adobe/Adobe Premiere Elements 15 is a directory folder.