Sounds to me like the hardware has failed. Let me guess, you dropped it and now it doesn't work? No. Let me have another guess, you dropped it in a puddle of water, and now it doesn't work? No. Oh, let me guess, you gave it to your child, who handed it back to you not working?
If mechanical buttons work, but the screen doesn't it sounds to me like the digitizer has been broken, and you need a new phone!