[Xanius]

Does it do it as soon as you turn the screen off, or is it turn the screen off wait a while then it won't come back on?
If it's the later then you're most likely getting the stand by bug and haven't updated to 6.1
If it's the former then you could always have a repair center check it out but in most cases that's not a hardware issue if it turns the phone back on, just will not resume out of standby.