If the vacuum pump is on and the the vacuum sensor is only reading atmosphere, or around 750,000 mtorr, first try to open the door. If you can open the door, then the vacuum pump is not connected correctly. If you cant open the door, make sure the seal has a large 1/2" black ring around the door. 

If the problem persists, try turning the machine on and off again. Sometimes the pressure sensor can hang and just needs to be turned on and off again. 

If that does not fix it, then it is likely that the pressure sensor or the buffer board needs replaced.