Corrosion, freezing temperatures, and shifting soil can crack copper or PVC supply lines and cause sudden pressure drops.
The plumber isolates the leak, excavates if needed, installs new piping, then pressure-tests the repaired section.
Pipe material, depth, and access difficulty determine how much the repair will cost.
Shut off the main valve and call (833) 770-0993 if you notice sudden pressure loss in San Andreas, CA.