The F stands for "frozen pipes."
The Sergeant shook me awake at an ungodly hour. "Honey, the water's not running."
Crap. Our low was below zero last night. The pipes were frozen.
Luckily most of them were not very frozen. Application of hair dryer to pipes got the water running everywhere again except the downstairs toilet.
Which is fed from a pipe that can only be reached from the crawlspace. Which I fit into more easily than the Sergeant, so in I crawled.
A misplaced piece of insulation was partly to blame. I pulled that away and ran the hairdryer over the pipe. Whoosh. The blockage melted and the pipe did not burst.
Good thing I'm not claustrophobic.