Your system has run out of application memory
I'm running into this when loading my project now that I've added all of my media... every single time I launch it. I started up task manager to see what was eating up my system memory, and when the system crapped out, task manager showed HitFilm consuming 55.98 GB of memory.
I'm running Yosemite on a MBPro Retina 15", 2.7 GHz model laptop. Has anyone else run into this?