What happened to Poloniex Exchange?

I'm curious to know, what exactly happened to Poloniex Exchange? It used to be a popular platform for cryptocurrency trading, but recently I've heard rumors about some issues. Is it still operational? Have there been any security breaches or regulatory problems? How have the users been affected? I'm interested in understanding the full scope of what's been going on with Poloniex and how it might impact the broader cryptocurrency market.