Hi Deepak,
Could you have changed anything with your screen? (for example if you are using a laptop, or multiple screens). Sometimes the tracker can loose the automatic screen detection. If so, it could be worthwhile to check this in the Tobii EyeX settings panel.
If the screen is unidentified it could post that calibration error message. This may not be the problem, as Showcase seems to work for you, but is is worth a shot.
/Mattias