Madhav,
    Gerry has the right idea.  I'd also add the following:
-can you see a change in vacuum once gas is introduced?
-I've seen in the past on our 252 that if the SE suppression cable is loose, all beams will fail.  So, check that you have the (I think) -165V or so on it.
-I've also seen that if there's a piece of debris on any of the cups, no beam.  I've had to flush out the cups with pure N2 before.  
-Ion source control board will fail.  Make sure all LEDs are green.  There are two red-one for a box/trap short, and one for ?? Look carefully, as all the green LEDs may not light either.  
-there's a reset button on the ISCB.  Resetting that may help.  Caution:  Turn off HV when dealing with this board! 

Hope this helps,
Ross