check the brake fluid level yellow cap bottle just behind the battery
check the fluid is between the max and min....
If low find out why its low.. does the brake peddle feel soft or go to the floor...
pump the brakes hard several times and get a mate to look under the car and round the back of the wheels for any leaks....
if in ANY doubt do not drive the car and get your brakedown company or a local mechanic to check it out
as above it could also be the handbrake release switch