Most vacuum uses one belt at a time. The belt connects the agitator (beater bar) to the roller which is driven by the motor of the vacuum in the base of the upright vacuum.
To check if the belt is present/intact on the vacuum, look at the sides of the base for two latches. Push the latches away from each other then lift up to remove the cover so that you can check or replace the belt. To replace the belt, lift the agitator out of the base (first lift the right side). Cut off the old belt with scissors if it is still intact before pulling on the agitator. You may need to pull the bumper forward to get the beater bar out of the base. To put on the new belt, slide it onto the roller of the motor then onto the pulley of the agitator. Replace the beater bar in the housing. Start with the left end cap and make sure to keep the arrow pointing down on both sides of the part. Make sure that the agitator turns and center the belt before reattaching the cover.
While you have cover off, check for anything wrapped around the agitator. Remove anything that prevents the beater bar from turning freely. Make sure the area around the agitator is clear of any clogs.
However, if the belt is missing, the volume of dirt picked up will be less but not zero. It is more likely that the unit is clogged. There are several filters that need regular cleaning or replacement. The motor filter is at the bottom of the vacuum bag cavity. It should be cleaned with warm soapy water then rinsed completely. Squeeze out the excess water and then let it air dry. Wait until it is dry before reassembling the vacuum.
An exhaust filter also needs regular cleaning. This is behind the dust cover near the bottom of the area covered by the cover. To remove the dust cover, pull forward and then lay it down. To clean, put the filter under cold running water. Then air dry completely before reassembling the vacuum cleaner.
Finally check if the hose is clogged. First remove the hose by grasping it near the base and twisting it as you pull it away from the vacuum. Pull the hose completely straight then plug in and turn on your vacuum. Stretch out the hose and then let it relax into its storage configuration. Turn off the vacuum and then unplug it. Check for and remove any clumps of dirt stuck in the hose before replacing the hose.
If you have cleaned the vacuum and it is still not picking up dirt, try a different pile height setting on the vacuum. Also check if the agitator should be replaced.
Manage My Life has the manual for the vacuum here: http://www.managemylife.com/mmh/owner_manuals/15123/KENMORE-Vacuum-Upright-L0807740?brand_name_search=33078
. This manual gives the pictures of the various parts that need to be cleaned or replaced regularly. Sears Parts Direct has some parts for the unit; see http://www.searspartsdirect.com/partsdirect/part-model/Kenmore-Parts/Vacuum-Parts/Model-72136078600/0582/0645000?sid=PDVertical
I hope this helps.