What is the make/year/model?
Possibly handbrake switch.
Probably low brake fluid level in the reservoir.
Brake fluid level will drop as the brake pads wear, so might just need topped up to the 'full' mark.
For peace of mind it might be best to get the brake system checked over for leaks.
If the car has a hydraulically actuated clutch it may be fed from the same reservoir, so check that system also.
flip
April 2009