Dear Gaurav,
Failure to reply of Notice gives AO the power to do assessment u/s 144 & once it has been done u cannot challenge it on the ground of validity..
Options for u -
1)U HAD REASONABLE CAUSE FOR NOT REPLYING TO THE NOTICE & THE SAME WERE TOLD TO AO BUT HE REFUSED TO ACCEPT..
2) AO has not done assessment properly, i mean the basic principles of assessment were not followed while estimating income, etc.
Validity of best judgment assessment cannot be assailed - A best judgment assessment can only be challenged in an appeal in respect of the quantum of income or the quantum of tax as determined by the ITO, and not on the ground of validity of the assessment - Gaurishanker Kedia v. CIT [1963] 49 ITR 655 (Bom.).