Empty or replace the vacuum bag. The vacuum bag is what holds all the debris the vacuum sucks up during your cleaning sessions. A full bag will not only block the air flow, but stop the vacuum holding any more debris.
Clean the rotating brush. The rotating brush can easily become tangled in hair, cords and other debris. There may be a complete blockade, preventing the brush from properly rotating or preventing proper air flow. Remove all such build-up.
Replace the drive belt. Following the vacuum's user manual, dismantle the machine to check the integrity of the belt. A new, properly working belt should have no holes, worn spots or misalignment. Replace the drive belt using the proper model number.
Clean and/or replace the filter, if your vacuum has one.