The game won't load at all. It keeps crashing when I press play from the launcher.

I had the same issue stating

Error initialising mixin config terrablender_forge.mixins.json

However I got around this by deleting the profile in my CurseForge launcher and then re-downloaded the pack and was able to get in.

In the future we need to know what version of the pack and we would need some kind of log to help. Thanks.