Turn off, battery out, pause, battery in, reboot.
If that doesn't cure it, you may have a failed phone.
You may get some mileage from :
http://www.askmefast.com/My_touch_screen_on_Samsung_GTS5230_is_frozen_how_do_I_get_it_work_again-qna1191089.html
acp
April 2017
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