[Also available on Reddit.]
If you suspect that the project you invested in is being grossly mismanaged or subject to a fraud, consider making a books and records demand. These demands, if made for a proper purpose, can force management to produce documents useful to either put a valuation on your investment or to aide an investigation into potential wrongdoing. Your rights and ability to make such a demand will be dictated by the operating agreement and/or the laws of the state your project was incorporated in.
If crafted correctly, books and records demands can provide valuable ammo useful in an eventual litigation.