Low water pressure can show up gradually or overnight, and the cause depends a lot on which pattern you're seeing.
If it's just one faucet or showerhead, the problem is usually local — a clogged aerator, mineral buildup in the showerhead, or a partially closed shutoff valve under that fixture.
House-wide low pressure points to something upstream: a failing pressure regulator, significant mineral scale built up inside older galvanized pipe, or a hidden leak that's bleeding off pressure before it reaches your fixtures.
A sudden, dramatic drop is worth treating with urgency — it can indicate a burst or badly leaking pipe somewhere in the system, especially if you also notice higher-than-normal water bills.
A plumber can check your home's static pressure, inspect the regulator, and if needed run a leak detection check to rule out a hidden pipe leak before recommending a fix.
Dealing with this in your home? Pipe Repair is available 24/7 across California.