After Effects cut out the first half of my timeline
I have a project I've been working on for a very long time, but had stalled finishing. After 16 days I finally continued working on the file, only to find that my timeline was quite literally cut out.

The project started in the middle of the footage I'd used, so I tried extending the composition and moving the clips further forward. Only problem is, every single one of them had been cut.

Normally, these would follow right after each other. Extending these clips would be a solution, but I'd have to adjust the time markers and alignment all over again.
I have one question: How do I prevent this from happening ever again?
PS: Let me know if you need more information.
Edit 1: The keyframes set for transitions have been moved aswell.
