Just discovered my total pension acct funds were paid out in 1996 and PBGC stated they sent a check to me, which I've never received. I just recently found out I had this pension acct from an old job back in the 1980's by research and was not aware the company I used to work at had a pension fund. The company has closed many years ago. Should PBGC be able to show proof of that check and that funds were cashed? PBGC's records show I still have the full amount in my acct, and they also show they already paid out my acct in 1996. What should I do ?

17 years is too long to require a company to provide proof of a check or where it was sent.
Some do, however, and will find it for a fee, because it means searching physical archives. Data is rarely kept in digital form (computer) that long.
If it ended up in the state's treasury file of unclaimed funds, that would be a lucky thing, and I would start there: Google '[your state] unclaimed funds.'