I would guess at a blocked ASI initially- remember it needn't only be caused by ice- insects are another possible cause. The indication staying rock-steady suggests constant pressure within the pitot tube- and I would normally expect some fluctuation of aispeed indication in normal flight. Then, when the turbulence hit, the stall may have allowed stagnation pressure to drop enough that the obstruction simply fell out.
The above is simply a possibility- I wouldn't disagree with any of the other suggestions.
As to teaching failure of the pressure instruments- I used to teach it at IMC level. I seem to recall failure of Gyro instruments was referred to as "limited" panel, and failure of the pressure instruments as "partial" panel.
Thanks for the account- food for thought for all of us I think.