There are a few things you can try.
1. Try holding down the "Power" button for about 20-30 seconds and then plug in the phone to charge. (we aren't looking for it to turn on, just charge.) If it does start charging, great. If not, then we try something else.
2. Next try a different charger cord, and also try your charger cord on another device. If your phones card works to charge another device, then that isn't the issue. If your cord doesn't, then you may need just to replace it.
3. From here I would try pulling the battery and then reinserting it. After you put it back in, try the above steps again.
4. It could also be the battery itself that needs replacing.
5. If none of the above work, you can try and take it to a local tech, or if it is still under warranty, contact the manufacturer.
6. Should none of this work, then you may need to just replace it.