The most likely cause is a bad brake light switch (if none of the brake lights illuminate) but if its only one of the brake lights, try changing the bulb. Even if they still work as running or turn lights because they have a double filament and may still work partially, but not as brake lights.
If you mean the brake lights are stuck on, then check the stoplight switch located on the top of the brake pedal.
brake switch ???
Usually that would be caused by a faulty brake light switch connected TO the brake pedal. If the brakes work the pedal is probably ok.
fuse, bulbs
BAD BRAKE LIGHT SWITCH AT TOP OF BRAKE PEDAL
check the fuse for the brake lights, they have thier own. also check the brake light switch at the at the top of the brake petal
Blown fuse or bad brake light switch?
Bad bulbs? Blown fuse? Brake light switch defective or out of adjustment
The bulbs are burnt out.
my back brake lights don't work, but the brake light on the back door does what could be wrong
You probably need to replace your brake light switch. It's near the top of the brake assembly, beyond a trim panel. The fact that all three brake lights don't work rules out problems in the multifunction switch (the cause of similar problems affecting only the two rear lights.)
Check your fuses. Sometimes it is just that simple.