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Try this ( a soft reset, which will not wipe data ):Hold down the power button.
Then tap on 'Reset'.
Then tap on 'Reset' again.
If that does not work for whatever reason, or if it just won't restart, try removing the battery for a minute or two, then try again.
If this doesn't address the problem properly, tell me more - whether you can use any apps, etc., and how old the Samsung device is. Thanks.