project needs rendering again after it was all rendered?

Okay I'm working in a project that is over one hour long and I finished color correcting all the clips and let that render a couple days ago. Today I open the project and all the clips need rendering again indicated by an orange bar on top of the entire project. What's up with that? It's not like rendering just takes a couple of seconds. This is going to take hours again. I'm trying this Final Cut Pro X but I'm still very uncertain about it. Seems the learning curve is taking me much longer than if I were just editing it back in the old standard Final Cut Pro. Any advice help thanks

Steve