This problem has just started to happen. Tekkit (and presumably all modpacks) ran fine last night. Today, I updated my Nvidia GPU and swapped from the 32 bit java to 64 bit. Upon launching, I found I could not launch any modpack what-so-ever. The launch would hide itself and after a moment, it would come back, having not even launched Minecraft. Vanilla and optifine work well, if that helps find the problem.
PasteBin logs from one failed run: http://pastebin.com/p1ANF4cM
If I need to put anything else here, please ask and I will.
Thank you.