Sadly there have been close to a HUNDRED exchanges go out of business in one way or another since bitcoin's birth. Often taking all coins with them. MtGox was the most famous but there were probably 10 more that year alone.
There's lots of different reasons for an exchange to fail. The owners could have planned it all along as an exit scam, or more likely the exchange had a vulnerable spot in its' security, and just got hacked. When you lose crypto, no FDIC can come to your rescue, so it's often gone with no chance of recovery.
I know some like Coinbase seem impossible to go under, but MtGox was the only big exchange of it's day in much the same way. QuadrigaX was the biggest Canadian exchange so far and it's been wiped out as well when it's founder "died." (More likely he faked his death.)
So stop leaving coins on exchanges! You've been warned.