Updated
Quick fix
Try a different cable, power adapter and wall outlet first. Then power the phone off and look in the charging port for lint. Restart the phone. Heat and battery-protection features can also pause charging on purpose.
Most charging problems are the cable, the adapter or lint packed into the port, not the phone.
Step by step
Swap the cable and adapter
Cables fail often, especially near the ends. Try a different cable and adapter that you know work, plugged into a wall outlet instead of a computer or power strip.
Look in the charging port
Power the phone off and shine a light into the port. Pocket lint gets packed in and stops the plug from seating. Gently loosen it with a wooden toothpick or a soft brush. Never use metal, liquids or compressed air jammed into the port.
Restart the phone
A software glitch can stop charging. Restart the phone, then plug it in again.
Let it cool down
Phones pause or slow charging when they’re hot. Take it out of the sun or a hot car, remove a thick case, and let it cool before charging.
Check battery-protection features
On iPhone, Optimized Battery Charging may hold the battery at 80% for a while and finish later. Many Android phones have similar battery-protection or adaptive charging options in Settings > Battery. This is normal.
Try wireless charging
If the phone supports wireless charging and it works on a pad but not the cable, the port may be damaged and needs repair.
Get it checked
If nothing works, or the battery is swollen, the phone gets very hot, or you see damage, stop charging it and have it inspected by the manufacturer or an authorized repair shop.