Have you tried a battery pull (leave the battery out for a couple of minutes) to see if this resolves the problem?
Unfortunately, that's one of the limitations of having security enabled - if the touchscreen stops responding, there's no way to enter the security key (Pattern, PIN, password) to be able to access the contents of the phone. There may be a way to accomplish that using ADB or the development environment but I'm not knowledgeable in that area, sorry.
You could try booting in Recovery mode (with the phone turned completely off, press and hold the Volume Up & Home & Power buttons simultaneously - once you see the splash screen you can release them all) and wiping the cache, but I'm doubtful that will help in this instance.
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