Try putting the belt back on. To do this, pull a black tab located on the gray divider between the brush roll and the two rollers. Lift the brush roll out and replace the belt.
If the belt continues to fall off, please take your deep cleaner to a technician. You can find your nearest BISSELL® Authorized Service Center by clicking http://www.bissell.com/support/product-support/service-centers or by calling 1-800-237-7691, available 24 hours a day.