My 2001 mercury grand marquis, when stopping at a light breaks went to the floor and the car accelerated which cause an accident.why are these cars loosing breaks and accelerating?

You have not shared what art of the country you live in, but I may be able to guess your answer.
All cars as old as yours are subject to brake failure, usually from leaking or rusted components.
A good inspection now should show you the rear brakes are bad, and perhaps the brake lines have rusted through.
Could also be a rear brake cylinder has failed, leaking out your brake fluid, and causing the brakes to give out when you needed them most.
Routine inspection probably would not have caught that, but having your brakes serviced every 7 - 10 years should have prevented it.
Lots of people do not do a complete brake service because it can get really expensive, though it would prevent your accident.
The acceleration problem is a whole different matter.
God bless your efforts.