You state you invested, but were not an owner, which implies a loan; is that correct?
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
Before you can take a loss you have to make every effort to collect the debt per the IRS rules AND the debt should have been in writing so in case the IRS was to audit the deduction you have proof of the debt and the ways you tried to collect.