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What maker and model # ***** phone do you have?
How old is the phone?
Try removing the battery for 2 days and 2 nights. Then put the battery back in, make sure it is thoroughly charged by putting it on the charger for 6 hours or more (regardless of whether it shows as fully charged already - new batteries need this to be done since they have been sitting idle on a shelf for a while, before they are first used...) and see if, after that, it works again.