Things are not that simple as even smart people do stupid things when greed kicks in and that's what happened here. While Luna has ~20% APY, Celsius was more moderate in that regard (I think BTC had 8-9%) so people automatically assumed that its legit.
Celsius model business was solid.
You lend people money in crypto, you make them have 120% collateral, and you charge the ones you lend money to more than you charge people depositing.
Unlike banks you have no fear of people getting unemployed and not being able to pay, but you have no problems with the value of the collateral as you don't issue mortgages that might turn 1/4 of the value, and more important you have a lot of clients with bad credit interested in short loans, clients who would pay 10% of in a month to get the funds needed fast.
In many aspects, the business was solid, more solid than being a loan shark or a bank or a pawn shop, but their problem was they tried to get more and they started using the money that was supposed to cover swings in collateral (as rumors say) to farm yields in different schemes, basically, greed kicked in, and the results, well, suck. I don't have a penny in Celsius but I know guys who had, and I can't blame them, normally you shouldn't have had a problem with it, of course besides the usually hacks and "hacks".
When you have $100 million worth of a particular altcoin, you cannot easily sell that altcoin at market rates, so if the price of that altcoin drops far enough to force many liquidations, Celsius will have no way of receiving sufficient money to cover the loan amount.
The halt in withdrawals happened at a time in which the price of ~all crypto was falling/crashing. This makes me tend to believe this is more of an issue of Celsius being a failed bank, and not them putting customer money in exotic "investments".
I'm one of the 1,7 million investors of the platform who injected over 5 billion dollars on their hands. Things aren't looking good, because there isn't transparency, the CEO has disappeared from social medias, all messages coming from them are evasive and say nothing enlightening, sock puppets accounts on twitter and reddit keep pushing an "us against them" narrative to protect Celsius, just like Mashinsky has always done and inexplicably CEL's price is doing well right now, pumping decently, what indicates the platform is burning investors' funds to boost their native token.
Anyway, they can't pull the rug without consequences. It's not an anonymous virtual investment scheme and there are too many people and too much money involved on it.
The lack of transparency is likely something that is being done on the advice of their lawyers. My guess is that any solution is going to involve money being injected from a third party, either in the form of debt or in the form of someone buying (part of) Celsius, unless they end up not being able to honor all deposits in full.