If you didn't receive any stock/shares, it would be a non-business bad debt. Deductible as a short-term capital loss.
If you received stock/shares, then it would be a capital loss, long-term or short-term depending on long you held the shares/stock.
You could have had shares, and not be in a public company. Privately held stock/shares are sold all the time.
Did you ever receive a Sch K-1 from the LLC? If so, was it a Sch K-1 from a 1065? That would indicate it was a partnership interest you held.
If it was a Sch K-1 from an 1120S, that would indicate it was treated as a Sub Chapter S corporation.
In any event, it would appear that it would treated as a capital loss....the question being short-term or long-term?
See this link
https://www.irs.gov/publications/p550/ch04.html#en_US_2016_publink100010713
https://www.irs.gov/taxtopics/tc409.html
