Not turning on when off

@chinedugoodluck coul be a lot of different things. You need to start with the beginning steps of troubleshooting.

First you want to check your power supply. Make sure it has the proper voltages. Check to make sure connectors are not bend etc. After that, check the charging port. Clean it of any debris etc, use a few puffs of air to clean it out. Make sure there is no corrosion, debris etc. that prevents it from working. If none of this works, get a ammeter and see if your phone draws any amperage from the charger.

You may also have to consider replacing the battery. A depleted battery can no longer be charged. Measure the voltage on your battery connector with the charger plugged in. You should get around 4.2V If you do not get this voltage If you don't get that voltage check your flex cable coming from the charging board. Make sure it is seated properly and does not have any tears or other damage on it. Your phone could have a separate charging board that could be faulty. Consider replacing that as well

Since you did not tell us the make and model of your phone, we really can't provide any further help to you.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.