When your iPhone won't charge it can be a result of a damaged charging cable, dirty charging port on the iPhone or even a hardware problem, software glitch or even a bad battery.
The most common reason why an iPhone and other phone won't charge is a result of a dirty charging port or even a bit of corrosion or lint in the charging port.
Also sometimes restarting the iPhone will fix the charging issue on the iPhone.
In some cases the charging cord and charger may even be bad and need replaced.
Or the the charging cable end for the iPhone might not be fully seated in the charging port of the iPhone.
You might also need to disable the optimized battery charging on the iPhone if it still doesn't charge with a good charger.
In some case a simple restart of the iPhone can resolve any temporary software glitches that could be preventing the iPhone from charging.
Also look on the charging cable and adapter and check for any signs of damage, like frayed wires, bent prongs etc.
And check to be sure that the outlet you're plugged into is getting power.
You can test another wall outlet and try charging the iPhone from a different outlet and if it still doesn't charge even when the outlet has power you either have a bad battery, software problems, dirty charging port or bad charger or charging cord.
The optimized battery charging feature can sometimes stop the charging to preserve battery health and so disabling it can sometimes help.