How do I report a loss due to reverse stock split that has not been reported to IRS, and hence not on form 1099-B?

I received cash in lieu for the fractional shares, and now I have no shares left.

Get your taxes done using TurboTax

You report it as a sale of stock with the cash in lieu being the "proceeds" of that sale.  If you really didn't get a 1099-B then use the "Stocks, Mutual Funds, Bonds, Other" interview and tell TurboTax that no 1099-B was received.

Tom Young