Expanding on @
Exedra's explanation, please post the modpack you're trying to launch.
Also, please do this:
Start the FTB Launcher, go to the Options tab and enable "Show Console?" and "Reopen launcher after exiting Minecraft?". Then repeat your problem and in the console after it crashes, select "Paste to pastebin.com" and then post the link here.
We are not magic wizards, do NOT expect us to do your work for you. Please go over the crash report yourself and research the problem, if that fails please return to us for help.