FTB (direwolf 20) dissappears a while after launch

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If I understood correctly, you launch Direwolf20 pack. It "Launches" but disappears after a while without opening right?
I found that(on my pc it was the reason) it was either a mod I added & removed after or a Re-Download of the entire folder after deleting it fixed the issue.
 
There was a severe problem during mod loading that has caused the game to fail
cpw.mods.fml.common.LoaderException: java.lang.OutOfMemoryError

You appear to have a 32bit computer or at least 32bit Java. If your computer is 64bit then install Java 64bit and uninstall the old 32bit Java version. If you have a 32bit computer then you are going to have a hard time running the 1.6.4 packs as is. The 1.6.4 packs require more than 1GB and 32bit Java limits you to 1GB.