Try unplugging it at the master, and see if the light goes off. If it does, the switch has been tripped. Either there's marginal leakage from your brake system, or it's passed a point of wear on the front or rear pads/shoes.
You have to fully unscrew the switch out the master cylinder (no fluid should be where it goes), check your reservoir levels, put the cap on the reservoir and then pump the brakes slowly and fully a few times.
While you're messing around, look at the switch for damage in the part which normally is embedded in the master cylinder. If it's OK, then carefully screw it back in place. Don't overtighten.
If it trips again, your rear brakes may need adjusting up (most likely) or there could be other issues to look at. Don't tax the car too hard befiore you're confident with knowing the cause.
Cheers, Adam.