Was marriage the basis for the conditional GC?
Though unlikely, is the i-551 stamp still valid? Any chance the original, watermarked i-797 receipt not past it's 12-mo/18-mo expiration date?
What were the reasons given to deny you boarding?
In a majority of cases, the i-751 is adjudicated within two years. Either approved, denied, or an RFE issued. You could check the status on USCIS' site using the case number found on the i751 receipt.
Received the conditional GC through marriage . Then got divorced so filed an I-751 on joint filing waiver which is still pending . I-551 stamp received on 03/2017 expired on 03/2018 . Original I-797 receipt- received dated 07/2018 - Says valid for 18 months . I Left US on Aug 2017 .
Did not provide specific reasons to deny boarding . They claimed to have contacted CPB / Embassy and were told not to allow boarding . Due to an expired G.C and the lack of a straight forward printed sticker visa on the passport or a transportation letter or a reentry permit , they were not too sure about a I-797 . Also looked at the travel dates and mentioned that I have been out of the country for more than 1 yr . Upon checking the status of the I-751 through USCIS it is still pending .