When you say "it locks itself", you mean it behaves as if you have tried to enter the incorrect pattern? Just want to be clear about exactly what it is doing.
In the absence of further information, I suggest forcing a shutdown by pulling the battery out. Leave the battery out for a couple of minutes to ensure that the RAM is cleared, then try again. If it still does it, repeat the procedure but this time boot into safe mode. If it works properly in safe mode the problem is probably a user app.
If it still does the same in safe mode then boot into recovery this time and see whether it will let you clear cache (I'm not familiar with Samsung recoveries). If you can, do that then reboot, and see whether it's fixed.
If it still does the same thing then we're running shorter of options. If the problem is software then a factory reset should clear it, but obviously you'd lose any data that are not backed up, so I'd keep that as a last resort. It's also possible that it's a hardware problem, e.g. a digitiser that's been damaged by a fall or by moisture. Did anything happen to it shortly before this started, and has the phone ever got wet?
Hopefully the simple, non-destructive suggestions at the start will be enough. If not, do post with as much information as you can supply and maybe we'll be able to work out something else.
