Almost certainly you've got a broken drive belt. This machine has two toothed belts and it makes sense to change both while you're at it. Incidentally, this is a job you shouldn't have to do very often - Sebo belts usually last at least 10 years.
So, here's how to get at the belts and change them... NOTE - any use of the words LEFT and RIGHT assume that you are looking from the REAR of the machine.
STEP 1. Separate the base from the bag compartment: press the button at bottom front centre of bag compartment and lift bag compartment upwards away from base. Sometimes this is quite stiff and four hands would be useful, you can lever it from behind with something relatively soft (the plastic handle of a dishwash brush works well) but take care to lift vertically upwards, not sideways or you risk bending a metal lever that's hard to get back into shape again. Rest assured that once you've pressed that button there is nothing holding the two parts together other than friction!
STEP 2. Turn the base upside-down and undo the FOUR DEEPLY-RECESSED SCREWS. All the surface-mounted screws are irrelevant to this job.
STEP 3. Turn the complete base right-way up, release the foot pedal and move the vacuum tubes out of vertical. The top cover will then lift off. You will see there are two belts - the pulley device between the motor and the brush roller is the sensor that measures the drag on the brushes and operates the automatic height adjuster to maintain constant drag. To the right of that double pulley is the electronic circuit board - note that it has a sensor that locates within the pulley device, be careful not to damage it.
STEP 4. At the left end of the motor cover, the drive cog is concealed by a small cover. Lift this cover out vertically, noting how it locates in the frame for when you come to put it back. Next, although not essential, it helps to remove the metal lever because if it falls out when you're not looking you may later be uncertain about how to locate the small coil spring. So slide that lever out towards the motor, noting how the spring locates.
STEP 5. Undo the two screws holding the circuit board (take care the washers don't slip away). Then undo the four screws holding the double pulley. Once that's free, you'll be able to change the belts.
Re-assembly is simply the reverse of the above process.
If you need more help, just ask. Please remember to rate this response.