There's a Capt. Obvious fix to check before you go any farther. Like many vacuum cleaners, the brushes on the Bissell carpet cleaners, by design, only spin when the handle is not locked in the upright position, so be sure to plug it in, turn it on, and depress the foot pedal to lower the handle while watching the brushes one more time to be sure that the fix isn't that easy.
If the brushes still won't spin with the handle in the unlocked position, lift the handle back up into the locked position, power off and unplug the Bissell, and look down and see if either of the two belts that make the brushes spin are broken, as a broken belt, or a belt that's come off the track, is the next most likely problem. Both belts must be unbroken and in the right place for the roller brushes to spin.
These belts can be seen in the clear plastic pieces on either end of the roller brushes. If you're having trouble locating them, **with the Bissell unplugged so the brushes won't suddenly start spinning and hurt your hand**, try spinning the brushes with your fingers while watching for the belts. They're black, about the thickness of two quarters, and the bigger one of the two (on the left end of the roller brushes, as you face the front of the Bissell), is just big enough to fit around both roller brushes. The smaller belt, goes around the back roller brush on the right and to a smaller cog wheel behind it. If you see a broken belt, or a belt off the track, that's your problem.
Here's a video you can watch for free on Bissell's website that tells how to replace a belt. I'm sure the belts can be ordered online somewhere or found at some Bissell dealers or repair shops. Note, again, the two belts are different. If only one is broken, be sure to order the correct one.
If the belts are intact and in the right place, does the metal cog wheel behind the roller brush right turn when the Bissell is on and the handle is dropped? If not, then the motor that turns that cog wheel must be burned out.