A question no one wants to hear is "Where's the hot water?" Any homeowner knows that this means something could be up with the hot water heater — but a few other causes may also be to blame. Some of these aren't bad issues, but they're not ones you want to deal with anyway, and one is definitely a problem that you have to repair now.
Clogged or Broken Pipes
The potential cause that is the most urgent is that you might have a hot water line that is clogged or a pipe that is leaking. If the hot water pressure is weak (in other words, if you turn on the hot water and get a weak flow, rather than a normal amount of flow that is simply cold), you could have a clog from debris that has gotten into the waterline. Given the amount of sediment that can build up in a hot water heater, a clog is not such a rare occurrence.
The other urgent issue is a pipe leak. If the hot water is really gone and the flow is weak, call a plumber immediately to repair the damaged pipe. If you haven't seen water gushing out of a wall, that doesn't mean you're in the clear; even a small leak that doesn't appear outside a wall can cause the hot water to disappear. A plumber can make the necessary repairs.
However, sometimes the water pressure is fine, and you just have cold water when you want hot water. High demand on the hot water supply in your home is the first potential cause to look at in this case. If you're trying to use the hot water in the shower at the same time someone is handwashing dishes in hot water and the washer is using hot water, that may be too much demand for your water heater to handle. You can get around this in two ways. One is to either replace the water heater or install another one (e.g., a tankless water heater dedicated solely to the washer), but you may find it easier to just be a little better about scheduling water use. Set a house rule not to use the washer when people are getting ready for work or coming home after an hour at the gym, for example.
Distance From the Water Heater
Another problem that most people don't realize is possible is the distance between the water heater and the fixture where you're trying to use hot water. If the water has to travel a long distance, like up three floors from a basement water heater, for example, the water can cool off substantially along the way, especially if the pipes don't have good insulation.
If you can't figure out what's causing the problem, call a plumber and have him or her take a look. Even if it's the weekend or after normal business hours, an emergency plumber can still make repairs and get your hot water working again. Look for someone who provides plumbing repair services in your area to learn more.