There are many potential causes for a rear light not to work. It could be a problem with the bulb itself, the fuse, the wiring, or a connection issue.
If the bulb is burnt out, then it will need to be replaced. If the fuse is blown, then it will need to be replaced as well.
If the wiring is damaged, it will need to be repaired or replaced. And finally, if there is a connection issue, it will need to be corrected.
In most cases, a rear light not working is due to a simple problem that can be easily fixed. However, if the problem is more complex, it may require the help of a professional.