Check if your phone is out of charge: it may have run out of battery, if holding the On button results in a flashing battery logo then this is the case, so plug it in and it will automatically come back on.
The phone may be on but the screen isn't working: The easiest way to check this is to hold the power button to shut it off and then turn it back on. If this doesn't fix it move on to the next solution.
If your phone was dropped or got wet: Your phone may have water damage. If it does, leave it to dry for at least 48 hours, ideally longer. Regardless of if this fixes the issue, it is best to bring it Techfixer store for one of our experts to investigate as, in the long term, your phone's inner circuitry will erode and the device will break.