Ok, so today after I had just installed a later version of Forge for a new mod, the forge file wouldn't launch. I went from Forge 1.6.4-9.11.1.916 to Forge 1.6.4-9.11.1.965. There was no crash report. When I pressed "play" on the Forge file, the launcher disappeared and nothing happened. Like I said, not even a crash report showed up. The mods I tried to use are: The Little Blocks Mod 1.6.4- 2.2.3.1, The Gulliver Forged-965-installer-0.14.3 and the SlimevoidLib-1.6.4-2.1.0.0. I decided to look in to it a little bit more and found this as follows:
Nov 04, 2014 10:30:34 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Nov 04, 2014 10:30:34 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Nov 04, 2014 10:30:34 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Forge Mod Loader version 6.4.49.965 for Minecraft 1.6.4 loading
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Java is Java HotSpot(TM) Client VM, version 1.8.0_25, running on Windows 7:x86:6.1, installed at C:\Program Files (x86)\Java\jre1.8.0_25
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2014-11-04 10:30:34 [SEVERE] [ForgeModLoader] Unable to launch
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.remove(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:114)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
INFO: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Nov 04, 2014 10:30:34 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
Nov 04, 2014 10:30:34 AM net.minecraft.launchwrapper.LogWrapper log
INFO: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Forge Mod Loader version 6.4.49.965 for Minecraft 1.6.4 loading
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Java is Java HotSpot(TM) Client VM, version 1.8.0_25, running on Windows 7:x86:6.1, installed at C:\Program Files (x86)\Java\jre1.8.0_25
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
2014-11-04 10:30:34 [INFO] [ForgeModLoader] Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
2014-11-04 10:30:34 [SEVERE] [ForgeModLoader] Unable to launch
java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.remove(Unknown Source)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:114)
at net.minecraft.launchwrapper.Launch.main(Launch.java:27)
alright. I'll see if that helps.