Check the calipers. If the calipers pistons are extended to far - this could cause the brakes to lock up. The pistons cannot be retracted.
The technique of braking short of lockup is known as threshold braking. Modern cars have anti lock brakes to help prevent from going into full brake lockup.
If drum brakes you probably have a bad wheel cylinder on the side that is locking up.
get a new car
Check the emergency brakes
Ether your low on brake fluid, or you have the wrong kind in it.
Defective master cylinder? Frozen caliper? Parking brake cable frozen?
Check your ABS system... How Anti-Lock Brakes Work Copy & Paste link http://auto.howstuffworks.com/anti-lock-brake.htm
by increasing the friction ABS brakes have the same friction as standard brakes, the way they work is to release the brake force to a wheel that stops turning preventing lock up and loss of steering control.
brake calibers might need replacing,the pistons might be getting stuck,its easy to replace.
Lockup - 2005 Life After Lockup 10-3 was released on: USA: 26 February 2011
Lockup - 2005 Life After Lockup 2 14-8 was released on: USA: 26 May 2012
Lockup - 2005 Lockup Raw Consequences 9-9 was released on: USA: 20 November 2010