Look through this section for info on the brake system, problem diagnosis, and repair - https://faq.ninja250.org/wiki/Brakes
It's not super complicated to work on, but some tools, patience, and mechanical knowledge are required.
Last season I had the rear brake of my SV stop working. After flushing and bleeding the system it has been fine. The rear brake should be used mostly as a supplement to the front brake - but it needs to work properly.