More than likely, you have a clog in the onboard hose. Make sure the vaccuum is unplugged from the wall. Remove the 2 phillips head screws on the back of the handle near the power cord inlet and pry the back part of the handle off. This will allow you to detatch the hose.
Then run a long, thin stiff rod through the hose - a half inch pipe works well, as would a narrow broom handle. Do this several times to remove all of the pet hair, dust, paper, etc. that is clogging the hose, then re-assemble the unit - your vaccuum should suck as good as new!
David Morgan, January 2007