Go to the Options pane of the FTB launcher, click the Show Console box. For setup reasons, you'll then need to run a pack (doesn't matter which one) and exit out of the launcher. Then, open up the launcher again. This should cause the console to open up. Try to launch your intended pack again with the console open. If it still freezes up, copy and paste the last 20-30 lines or so from the console from where it stops working (and/or the last big block of red text if it exists) into a new post on this thread. Make sure to enclose it in a spoiler tag; once you've pasted the text, place [spoiler] before it and [/spoiler ] after it (this hides the text until a button is clicked, enhancing readability of the forums.) You can also click the button on the console that says something like "paste my log to pastebin" and link the paste on pastebin here.