Check the console, if it says "Failed to load class org.slf4j.impl.StaticLoggerBinder." download the file i attached and put it into your modpacks folder (I found out how to fix that from http://www.slf4j.org/codes.html#StaticLoggerBinder). But if it also says "File version.json not found" Then I can't help you.
slf4j-simple-1.7.12.jar