in short: the cold wallet stores most of the bitcoins that were deposited. it is not accessible by the trading platform, so if anything goes wrong (like the trading engine incident few days ago), those coins can not be withdrawn from people who profited from corrupted trades. also if someone were able to hack the system, he cold not access the cold wallet. so the biggest part of coins is safe form incidents like that. the hot wallet is the wallet from which btc can be withdrawn. if its empty, it gets refilled by btc24 with money from the cold wallet. he usually did that every hour.
so the maximum that can be lost is the hot wallet content (usually some hundret btc). the minium that can be lost is 0 btc.
the other thing is, that there seem to be legal problems with the account being block etc.
check this thread, because this is where the main discussion is taking place:
https://bitcointalk.org/index.php?topic=174600.0