A brake pedal that is suddenly hard - requiring much more foot force to stop - is the opposite of a soft pedal, and it points to a specific system: the power assist. Here is why the pedal goes hard and what to check.
Lost vacuum assist
Your brake booster uses engine vacuum to multiply your foot pressure. If it loses vacuum, you lose that assist and the pedal becomes hard and high, taking much more effort. The brakes still work - you just have to push far harder. This is the most common cause of a hard pedal.
Common sources of lost vacuum
Look at the vacuum supply: a failed check valve that does not hold vacuum, a cracked or disconnected vacuum hose, a leaking booster diaphragm, or on some engines a failing vacuum pump. A classic clue: the pedal is fine while the engine runs but rock-hard right after you shut it off, or on the first stop at startup.
Cold-morning hardness
If the pedal is hard mainly first thing in the morning and softens as you drive, a check valve leaking down overnight is a likely cause - see brakes hard when cold. The engine rebuilds vacuum once running.
What to do
A hard pedal is unsafe because stopping takes much more effort - diagnose the booster, check valve, hose, and pump. It is not usually a pad or fluid issue. Get the assist restored, and keep the friction side healthy with quality pads and fresh hardware.