Yeah I would just start with a general inspection then. Lots of good things to check already mentioned here, just look at the condition of everything. If nothing is obviously stuck, leaking, the pads and shoes don't look fouled or burnt etc then move on to a good brake fluid flush out and refill, check to make sure everything is moving as it should and work from cheap/easy to expensive/hard. One thing at a time so you know what fixed it. Fluid is cheap, brake hoses are cheap, pads and shoes are a bit more but are wearing parts anyway etc. If after that stuff the pedal is still rock hard and the car doesn’t stop then look into the master cylinder, the seals in them can stick, swell, etc.
