Hi
Follow the steps here, as appropriate, including checking the charging port on your iPhone for any debris or lint:
- If your iPhone, iPad, or iPod touch won't charge - Apple Support
- If your iPhone, iPad, or iPod touch won't turn on - Apple Support
Ensure also that you are using either a genuine Apple cable or an Apple-certified / MFi cable. If you are already doing so, try instead using another such cable. Apple also recommends using genuine Apple USB Power Adapters.
If necessary, contact Apple Support, make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance.