More information needed.
Did the pedal go to the floor? If this happened suddenly it is almost certainly hydraulics - if the fluid has not gone down it has to be the master cylinder.
But if the pedal did not go to the floor, and if conditions were wet, it is almost certainly just wet brakes. People who have only driven with discs have either forgotten or never knew that wet drum brakes do not work. Usually just rain, no matter how heavy, will not wet them, but driving through even just a few inches of water is likely to. The procedure is to gently apply the brakes for several hundred yards until they dry out and start to bite after going through water.
Other possibilities could be that an adjuster has slipped (have to be worn to do that) or (unlikely) a lining has come loose, or a severe oil leak onto the brakes on one wheel, although this is not normally sudden, and you would still have brakes on the other wheels - all leaking at once is not believable.
If it has power assisted brakes, the problem could be loss of vacuum (but this would only increase pedal pressure, unlikely to be interpreted as loss of brakes), which is almost certainly due to a hose leaking or off, but could be the diaphragm on the booster has gone. If it is a diesel with a butterfly on the intake to provide vacuum, then it could be the linkage to this is off.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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