There could be a number of reasons for this. First, make sure you’ve given the pack more than 3gb of RAM and installed a 64bit Java. If you’re unsure if this is the case, then please follow this video: https://www.youtube.com/watch?v=kjpYsH2jWoM
Note, if you’re using the Kinetic Launcher, it will come with 64bit Java. You don’t need to install it. Just press the settings cog, and make sure you’ve given more than 3gb.
If this isn’t the issue, then you’ll need to grab the crash report or log. To know how to find a crash report, check this page out here. If you don’t know how to read a crash report then please contact our staff team via our support forums here