Turn the power switch to "Off" and unplug the vacuum, and then turn the vacuuming attachment over if something has become jammed in the bottom. Remove the obstruction with your hand and continue. Press the reset button if you're using the larger power nozzle.
Check the fuse and replace it with a new one or reset the circuit breaker if the vacuum won't start at all. Ensure the plug is securely in the wall outlet.
Inspect the filter bag for an obstruction if the dirt isn't being picked up when you roll over it. Change the vacuum bag if the vacuum shuts off and the indicator light is flashing.