Eligibility to ITC

Hi baskar

180 days limit applicable. 180 days due date will fall in october so if you have already taken input and making payment now then technically you have to reverse in oct 3b and again claim in oct 3b so effect is nill

if amount is huge then give effect of reversing and again claiming. if Amount is less tgen skip thos compliance

As per section 16(2) (a)  of the CGST act 2017  you must  have valid  GST  Tax  Invoice with correct  GSTN no of  the   Supplier   to claim  Input  Tax credit .  As you   mentioned  in the query that  you have lost  Tax Invoice so  you can  not claim Input Tax credit . 

Better to get invoice from supplier