If you suspect that you have a problem with your ventilation, then you might want to take a look yourself before calling a professional. While you might not be able to actually remove the blockage yourself, you can definitely narrow down its location.
How to Find a Blockage
First of all, you need to identify which vents are functioning correctly and which are not. In order to do this, you should run the central fan and then check each vent. If you can feel air blowing through a vent, then the path from the central fan to that vent is likely clear. If you cannot feel any breeze in some vents, then the blockage is likely somewhere between those vents and the fan. If multiple vents are affected, then the blockage might be pretty deep and difficult to access for the average homeowner. If you cannot actually feel a difference in air flow when the fan is on or off, then you can verify the problem by trying to use the AC or heater for a bit.
In order to do this, you should close up every window and door possible. Then, run either the AC or the heater for about an hour. After that hour, check the temperature in every room. You don't need to take an exact reading, but you should record which rooms were affected and which rooms were not. If a room has a vent but is not getting hot/cold air, then there might be a blockage that you need to worry about.
Checking the Vents
Once you have identified the vents that are causing problems, you can try to take a look at the problem yourself. For this, all you need is a flashlight, a screwdriver, and maybe a ladder, depending on the height of the vent. Open up the vent cover with the screwdriver and take a look inside with the flashlight. If you can actually see the blockage, then there is a good chance that you can reach it yourself. If you can't see the blockage, then you still might be able to reach it if you are very careful.
To probe or try to remove any blockage, use a long, thin, lightweight object, such as the handle of a broom. Ideally, you want a hook on the end of whatever instrument you end up using. Try to feel around for the blockage if you can, but be careful not to be too forceful. You don't want to damage your ventilation by accident.
For professional help, contact a company such as Capital Heating & Cooling.