Why won't my MacBook Pro charge when plugged in?

If you have had it three years, then I'm assuming it is some sort of USB-C model. Having the exact model is really important since some computers may have known issues specific to that single model. You can get the exact model by clicking the Apple menu and selecting "About This Mac".

Try an SMC Reset (read the article carefully since the procedure is different for laptops from 2018+ which use the T2 security chip and only applies to Intel Macs).

Try unplugging the charger from the electrical outlet for a minute to reset the charger's internal circuitry.

Disconnect all external devices in case one of them is causing a problem.

Make sure to connect the charging cable directly to the laptop. Multi-port adapters, docks, and hubs may not allow the full power of the charger to reach the laptop.

Try connecting the charging cable to another USB-C port especially on the other side of the laptop if you have four ports.

Try rotating the charging cable's USB-C connector 180 degrees upside down which may work if the USB-C port is half bad (indicates a hardware issue).

Make sure you are using an actual USB-C charging cables. Not all USB-C cables will charge a laptop.

Here is an Apple article for troubleshooting USB-C charger issues:

If your USB-C power adapter isn't charging your Mac notebook - Apple Support

You may need to have Apple or an Apple Authorized Service Provider examine the laptop to provide you with a repair estimate.