This is a major risk with exchanges, as they’re a very attractive prospect for hackers. One of the most famous incidents was the closure of the Mt Gox exchange in 2014, when hackers made away with around 740,000 bitcoins, which was 6% of all existing bitcoin at that time. If this were to happen, it is very likely you would permanently lose any crypto you have on the exchange. This is why it’s recommended to keep your coins in a personal wallet if you’re not intending on trading them regularly.