Hi, welcome to the site!
Primarily, the brush control (the grey knob on the head of the vacuum- bit you kick down to start vacuuming) should be set on the carpets setting. This setting allows the brush to spin, however the brush will not spin when the machine is in the upright position.
I presume you have removes the soleplate to remove the guitar string? If so, many people do fit it incorrectly, some people take the brushbar out of it's housing so this results in the brush not being fitted correctly. The end caps of the brush must be fitted securely in place so that the soleplate can be fitted without forcing it to fit on. Check if the brushbar spins fine by hand.
If the brushbar spins fine by hand and fails to spin, the belt may have slipped or snapped. It's pretty easy to change the clutch or replace the clutch belt.
1) Remove the soleplate by turning the light grey plastic screws with a flatblade screwdriver or a coin at a quarter of a turn anti clockwise.
2) Remove the brushbar off the belt by turning the brush and pulling it out at the same time.
3) Remove the side internal hose. It pulls off at the end with a grey cuff and twists off the opposite end at the brush housing.
4) To remove the brush housing, you need to unclip it off the cleaner head. To do this, rock the housing up and down whilst pulling it away at one side and then repeat with the other side. Alternatively, you can pop it out with a flat blade screw driver shoved in between the housing and the cleaner head at both ends each.
5) Remove the U bend at the bottom of the back of the vacuum (the clear plastic tube which can be unclipped by the grey clip just by the bottom cable hook of the vacuum.
6) Remove the 3X T15 screws holding the black clutch cover in place and remove it.
7) You should now see the belt, it's either snapped or slipped. This rarely happens but in your case this could be the problem.
The clutch can be removed by popping the clutch knob off and it can then be pulled out with the big belt (be careful that it doesn't fly out and knock into your knuckle as it springs out by the tension if the belt).
Hope this helps! Any questions please ask