What could cause my brakes to not work?

With the symptoms you are having it sounds like you might have a problem with air pockets in you brake system or low on brake fluid. Have you had your brakes looked at? Might have worn brake pads also

first thing I would do is check your brake fluid make sure its full if not check for visible leaks

also check your brake linings might just need a set of new brake pads

Bas/ - brake assist, could be a faulty brake switch located on the brake pedal would be better to scan the vehicle and check the code

ESP/ - electronic stability systems, common problem is a steering angle sensor. Best to scan vehicle