Phone won't charge
This is really common, and it's often just the cable, the plug or a bit of lint in the port.
Be careful: If the phone is swollen, very hot, smells of burning or got wet, unplug it, don't charge it, and take it for in-person help.
What to do
Step 1: Check that the plug is pushed all the way into the wall and the outlet has power.
Step 2: Try a different wall outlet, then a different cable and plug if you have one. Cables wear out more often than phones do.
Step 3: Take the phone out of its case, then push the cable firmly into the phone.
Step 4: Shine a flashlight into the phone's charging hole. If you see lint or dirt, ease it out very gently with a wooden toothpick. Never use anything metal.
Step 5: Leave the phone plugged in for at least 30 minutes. A completely dead battery can take a while before the screen shows anything.
Step 6: Restart the phone while it's plugged in. If it won't respond, see our answer on force restarting a frozen phone.
Step 7: If your phone can charge wirelessly, try a wireless charger to see whether the problem is the port or the battery.
Still stuck? If it still won't charge with a different cable and plug, the port or battery probably needs fixing, so take it to the maker's store, your carrier or a trusted repair shop.