Unplug the shredder... Take the top off and turn it upside down. Use a screwdriver to take the lower cowling off and look at the motor. If the mounting area where the motor sits looks melted at all, the machine is toast. If the motor mounting area looks good, look at the slot where the paper is inserted... there should be a sensor (that "sees" the paper when you insert it) at the paper slot. Check to see that the sensor isn't damaged.
These shredders have a "paper in" sensor. This is either optical or mechanical. Sounds like the sensor is blocked and therefore "thinks" there is still more paper left, and this is why it keeps going.
Before you do anything UNPLUG THE SHREDDER. The sensor is located about in the middle and at the level of the blades. If it's mechanical, you should see a little finger, else it is electronic. Shine a light in the area and you should be able to spot where the blockage is. A shot with compressed ais usually gets rid of the offending particles.
After you're finished working on it, set it back securely on the basket and plug it in.