Being stuck with a broken heater or AC can be a huge pain, especially if the climate isn't particularly temperate. However, what might appear to be a problem with your heater or AC might actually be a simple and easily fixed problem with your ventilation. To diagnose the situation, here's what you should do:
Determine the Extent of the Problem
The first thing you need to do is figure out which rooms are affected and which are not. If you notice that some rooms aren't getting hot/cold air while others are, then your problem is almost guaranteed to lie in the ventilation.
Turn on either your central heater or air conditioner and close up as many doors and windows as you can. This will help isolate the problem and make your diagnosis a lot easier. After an hour or two of running the heater/AC, you should check the temperature in each room and record it. Now, you don't need to write down exact temperatures, just whether or not certain rooms are drastically different temperatures than the rest of the house.
If you do find that some rooms have outlier temperatures, check to make sure that they have vents in those rooms. If they do, then this can help you narrow down exactly where the problem is in your ventilation. The more rooms that are affected, the deeper the blockage is in your ventilation.
Try to Fix it Yourself
Once you have narrowed down the rooms in question, you should take a look through the vents in each affected room. You might need a ladder, a screwdriver, and a flashlight in order to effectively do this. Open up the vent and looking inside with the flashlight. If you can spot a blockage, then you can likely reach it if you have a long, thin object such as a broomstick or cane. If you can't actually spot the blockage, then you might need to call the pros.
Get Professional Help
If you can't reach the blockage yourself, then there is nothing wrong with calling for backup (from professionals such as those found through http://www.borterheating.com). The pros will usually bring specialized vacuum cleaners with extremely long hose extensions. These vacuums allow them to reach deep into your ventilation system and clean out any problems. As an added bonus, this often results in a significant amount of dust removal in your vents as well. On top of that, you won't run the risk of accidentally damaging your vents by probing around on your own.