How do I fix my 4 slice toaster. Half of it works, the other doesn't

See if it has thermal fuses once you open it up. unplugged of course. Check the side that does not work for continuity of that fuse. Replace it if it is open and check the location to see if it is located too close to an element. Either crimp using copper brass or steel crimps or use ceramic wire nuts to attach the new one.

Also look for a broken element. Depending where the break is, sometimes you can make it functional by overlapping the broken area, and that will make decent contact, or sometimes you need to add a metal ferrule and crimp it down.