Sony experia st18i blank screen problem

I have Sony experia St18i which have a very strange problem.
phone is working very fine till I lock the phone with lock button.
when I lock the phone screen goes blank and I am able to do call’s and screen touch is also working fine.when I do not lock the phone it’s work perfectly.I can’t understand what to do.
sorry for my English

please give my any solution Thanks.

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