You need to scrutinize all documents associated with the initial investment to get a clear understanding of just what kind of authority the management of Company A has under its agreements with the shareholders. You might be surprised at what they are able to do without first getting approval from any of the other investors.
If company B acquired Company A, they would only be able to do that by buying the stock, whether it was publicly traded or privately held. If Company A made some side deal to sell out to Company B and just kept the proceeds rather than distributing to the shareholders then it sounds like some sort of embezzlement may have gone on.
Company A sold its assets to Company B. A was not acquired by B. Company A still exists as an entity. A’s assets would consist of whatever good and valuable consideration it received for the sale of its assets to Company B. Presumably, as a shareholder of A, the investor owns a share of those assets. If Company A divested itself of whatever assets it received for the sale without compensating the investor, then the investor bought something different than what she understood it to be, or there has been some malfeasance.