If you suddenly notice that your furnace is not firing and your home is getting colder just when you need it most, or your hot water has ceased to function in its one job, and your home was built after about 2005, it could be that your high efficiency furnace's intake or exhaust is plugged up. Here's what to do:
First, go outside and check for white pipes about the size of your forearm coming out of the basement wall. They are usually located on the side of the home, but could be the back wall, usually below waist level. If you have them, then either hot water heater or furnace, or both, are high efficiency models.
Check these pipes to see if there is frost buildup (usually intake) or giant icicles (usually exhaust) that are plugging them up. If there is, then you'll need to clear out the pipes so the air can flow freely. I've found that the frost is fairly easy to clean out of an intake, as it just requires a gloved finger to get rid of frost. If there are icicles hanging from the exhaust, it's best to whack the icicle, not the pipe, if possible. You don't want to crack or break the pipe.
If that was the problem, then after clearing, the device may fire back up. If not, then it's time to call the service people.
Hopefully this will help you.