My tracks have disappeared?

Hi there,
I’m fairly new to editing and have been able to get this program to do what I need but recently I’ve had two instances where after saving and closing a project, when I reopen it, three tracks are gone. The play back window shows my video but it won’t allow me to export it and I can’t edit anything. What am I doing wrong?

I am not sure, never heard of that before or experienced it. Do not use File > Open MLT XML as Clip to open a project for editing. Just use Open or double-click it in the list of Recent projects.