What causes sudden low household water pressure?

There is several causes that may be responsible for loss of pressure in entire house. Among the most obvious causes, are:

1. Faulty pressure regulator
2. Main water valve on side of the house is half way closed
3. Main water meter is partially closed at the curb
4. City works on water main somewhere in your neighborhood

Have you had any plumbing work done on your house recently ?

Are you on a well or city water system?

City water that comes eventually from municipal wells.