Check Your Connections
Firstly, check the obvious points. Did you definitely remember to charge your phone? Has the charger become partially dislodged from the wall socket? And is the power socket turned on?
Dirt and Dust
Next, ensure there's no dust, dirt, or other gunk clogging up either the USB port on your charging cable or your phone's charging port. Remember, the connections are delicate. The best way to remove grime from ports is to use a can of compressed air.
Check Your Battery
If your phone died and won't turn on or charge, but you are confident that your charging equipment is working and a power cycle hasn't solved your problems, it's time to turn your attention to your phone's battery. It might be the reason your phone is not switching on.
Most phone and tablet batteries start to rapidly decline in quality after a few years of use. Eventually, they will completely die. Other types of damage can also irreparably affect the battery. Liquids, falls onto hard surfaces, and extreme temperatures can all render a battery useless.
Regards,
Will