You know when those economists say 'the higher the demand, the higher the price. The lower the demand, the lower the price'?

All those investors bought shares with money, right? Say, one guy bought a share worth 100$ 3 months ago, from someone else. Aight, well, everyone is freaking out right now and getting sick, and he needs some goddamn toilet paper. He needs money to buy the toilet paper. So, he wants to sell his share. But it's not just HIM selling his share, it's thousands of people selling their share. All these people want their goddamn toilet paper, and they all have the same idea: sell their share. There isn't enough demand to buy all these shares tho. Nobody has the money to buy shares, everyone has to buy toilet paper. So, the price of shares fall.

He isn't going to sell his share for 100$. Why would anyone buy a share for 100$ if other people sell their share for way less? So, he has to lower his price as well.

The share that used to be 100 is now worth, what, 10$? That single share lost 90$ in value.

It's a lot more complicated than that, but this does explain 'where' the 90$ goes to. It doesn't 'go' anywhere. Something used to be worth 100$, but nobody wants to buy it now, so they have to lower the price. Now it's 10$, which means it lost 90$ in value.