if revocation application has been rejected then what is the procedure for re opening

Reply—As per section 30(2), the proper officer may,  in such manner and within such period as may be prescribed, by order, either revoke cancellation of the registration or reject the application:

Provided that the application for revocation of cancellation of registration shall not be rejected unless the applicant has been given an opportunity of being heard.

After perusal of above provision, it can be said that the proper officer has power to reject the application made by applicant for revocation of cancelled GST registration provided a reasonable opportunity of being heard has been given to the applicant. Procedure of re opening of rejected application is not available. 

Therefore, if department has not given reasonable opportunity of being heard to your client, an appeal can be file against department.