It's 4 degrees here and supposed to get down to -9 tonight. I have lots of cold water but no hot water. Can hot water pipes freeze? If so, why haven't the cold water pipes frozen? I have opened all of the cupboard doors, turned the heat in the house up and opened up all of the hot water faucets in the house. One faucet has water coming out of it, but it isn't even a little warm. How do I get the hot water to start flowing again? Thanks!

As strange as it may seem hot water pipes will freeze before cold. It does so because there is more air in hot water than cold.
Apparently you have pipes that run in an unheated area such as a crawl space underneath you house. Nothing more you can do at the present time to restore hot water unless you get up under the house and warm the pipes with a hair drier.

Be aware that you may or may not have a busted pipe. You won't until the pipes thaw.

Thanks, that's what I suspected.

You may want to install heat tapes on any pipes that pass through an unheated area.

I just had the same problem, and luckily was able to thaw my pipes BEFORE they burst! Leave the hot tap open, so pressure can release, and get under your crawl space if you can. If you have a sense of the general area where the frozen area of pipe may be (if one faucet is frozen but another isn't, for example) you can try to find the source by looking for any exposed (non insulated) pipe, and feel along it with bare hands for the coldest spot, which will likely be closer to the house exterior and colder air. You can then heat the exposed pipe with a hairdryer, heating pad, hot water bottle etc held to the pipe (do NOT use an open flame like a propane torch!). If finding the source isn't possible, you should make sure all crawl space vents are covered-- with wood scraps or even cardboard if need be, just so long as the fit is snug and drafts aren't working in. Then put a small space heater (use a modern, safer one that blows warm air but doesn't have parts that get hot or may start a fire) in the crawl space, blowing in the general direction of the pipes. Mine thawed within a few minutes, but it could take a few hours. Check both the tap and the heater periodically, the tap will start to drip as it thaws. Also, be aware that if the pipe DID burst, it will leak once it thaws, so be prepared to shut the water off at the main or nearest shutoff valve in case that happens. Good luck!