Yeah, I'm really new to bmx, I bought a mongoose fuzz extreme just to get started cause I didn't want to spend that much money at first. But I seemed to have really loosened the back brake cable somehow that it's loose at the lever and when I pull the lever, it doesn't retract. I plan on taking it to my lbs this week to get the brake fixed, how much do you think that would run me?

If you don't have the money check out Park tools web site. https://www.parktool.com/repair_help/FAQindex.shtml

How did the brake loosen? Did you try to adjust it or did you wreck?
It could be something as simple as tightening the cables which most shops (at least in my hood) wont charge you for or you may need a new cable and pads which could run you about 10 buks or so.
Hopfuly you wont need a new brake system.

What kind of brake is it? Is it sidepull, center pull, linear pull being that it sounds like a dept. store bike it may be a build issue.

ok, I think the cabling just got loose, and the end of the wires weren't straight (real loose), so I couldnt get it back into the screw, so I think I need a new cable

just go no breaks that wat i did with my first bike breaks they were awful and i never had time to get new ones but me what do you need breaks for besides to stop