What happens when one of the 3 Phase load is disconnected suddenly?
Your question title is this: -


  What happens when one of the 3 Phase load is disconnected suddenly?


And the pictures in your question (ignoring the line break symbols) are for three phase systems with an asymmetrical load. Your specific question is this: -


  what will happen when one of the three phase loads get disconnected
  just like the in the figure below?


So, for me it's reasonable to assume that you know how to calculate the neutral current with the imbalanced load intact. If you do and it seems reasonable that you do then, for part (a), calculate the neutral current with Z2 set to infinity.

For part (b) it's a similar story just derive the formulas for the three currents and raise the YB impedance to infinity.

If in fact you cannot calculate the currents before the lines are broken then you are asking the wrong question and you should walk before running.


  For (a) which is star-connection, is the phase voltage is still
  Vline/sqrt(3)? Is Ineutral= -(IR+IB) still valid if IY=0?


In the absense of any other knowledge about the 3 phase supply it has to be assumed that it is perfect and therefore the \$\sqrt3\$ relationship holds true always. Neutral current is (under general circumstances) IR+IB+IY but you have to take into account phase angles and therefore some current cancellation.


  For (b) which is delta-connection, is the phase current of still
  Iline/sqrt(3)?


For an asymmetrical load no, the relationship does not hold-up. That relationship is only valid when the load is balanced.