1. The battery may just not have enough charge. Make sure to plug in the phone with a known working cable to ensure that the battery has enough charge.
2. If you are using a known functioning charge cable, it’s possible that the battery is dead. You may need to replace the battery.
3. The power button may be stuck, and need replacement.
4. The screen cable may be dislodged or is faulty. If it is firmly seated in the slot, then the cable may need replacement.
5. Motherboard is faulty. It will need replacement.